Emergency services were called to Brent Civic Centre in north London at 2.25pm and found the victim, believed to be a teenage boy or man in his early 20s, with stab wounds.
He was treated by paramedics but pronounced dead at the scene.
The civic centre, home to Brent Council's headquarters and a library, was evacuated and is now shut until further notice - with police yet to make any arrests.
The Metropolitan Police are also "urgently" trying to identify the victim and inform his next of kin.
Staff, residents, and visitors all witnessed the attack and were left "shaken", according to council leader Muhammed Butt.
Police have enacted a Section 60 order, giving officers enhanced powers to stop and search people and vehicles.
Superintendent Chris Clarke, lead for emergency response policing in the area, said: "An investigation is under way to establish exactly what has taken place and trace those responsible.
"We recognise the concern this incident will cause within the local community and are committed to keeping residents informed as our inquiries progress.
"In light of the incident, we have authorised additional stop and search powers to help prevent further violence, support our investigation, and keep people safe."
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Mr Butt urged people to stay away and praised the emergency services for their response.
He said: "Today, Brent has been shaken by an appalling and deeply tragic incident at the civic centre, in which a person has lost their life.
"Our thoughts are first and foremost with the victim, their family and loved ones, as well as the staff, residents and visitors who witnessed what happened.
"The civic centre has been evacuated and will remain closed until further notice."
Police were called to reports of a blaze at a property on Berkeley Avenue, Hounslow, soon after 1am on 19 August.
The Metropolitan Police say Anna Kowalska, 50, died in hospital following the blaze, adding her next of kin have been informed and are being supported by specially trained officers.
Another woman, 40, remains in hospital in a life-threatening condition, while six men and a woman have been discharged from hospital after being treated for minor injuries.
Slawomir Alama, of Hounslow, has been charged with murder, as well as eight counts of attempted murder.
He has also been charged with arson with intent to endanger life, possession of a class B drug and possession of a knife in a public place.
The 41-year-old was arrested in Leeds on Tuesday, the Met said, where officers from London were supported by West Yorkshire Police.
Alama has been remanded to appear at Willesden Magistrates' Court on Wednesday.
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Pawel Kupla, of no fixed address, was charged on Monday with one count of murder, eight counts of attempted murder and one count of arson with intent to endanger life in connection with the incident.
The 34-year-old appeared at Willesden Magistrates' Court on Tuesday and will next appear at the Old Bailey on Thursday.
The Canadian government said the counter-tariffs take effect from 8 September on what it said was $20bn (£17.1bn) worth of US goods.
It will impose duties of 15%, 25% and 50% across 700 products, including steel and appliances, imported from south of the border, to match dollar-for-dollar new duties imposed by Washington.
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Everyday items like seafood, cheese, clothing, cosmetics and toilet paper will be impacted as one of the world's largest trading relationships is hit.
Vital manufacturing components: steel and aluminium will see their tariff rate double from 25% to 50%.
It comes after talks broke down between the sides last week, and the Trump administration implemented planned tariff increases on Canadian goods on Saturday, including alcohol, wood products and clothes.
Further tariff hikes on steel, cars and car parts were announced by Mr Trump on Monday, due to be introduced in January.
The impositions are likely to be costly for businesses and consumers in both the US and Canada, making goods cost more and sparking a round of inflation and possibly economic contraction.
Not just a trade war
A war of words had broken out between the countries.
Doug Ford, premier of Ontario, Canada's most populous province, threatened to cut off electricity and mineral supplies to the US on Monday.
Mr Ford said that Ontario powered 1.5 million homes and businesses in the US - and he warned that Mr Trump "better have a pack of batteries".
He added: "You won't get a grain of sand out of Ontario."
Mr Trump hit back on Tuesday, suggesting he could rename Lake Ontario as Lake America.
"I deal with many countries, and Canada is easily the most difficult and unreasonable. They feel entitled, but they are not a State, and will be entitled no longer!", Mr Trump said on Tuesday.
Canadian minister of finance François-Philippe Champagne described US demands as "uneconomic", "unfair" and
"unacceptable".
Police entered a training centre run by UNRWA, the agency that supports Palestinian refugees, ordering the staff to leave and take control of the facility, known as the Qalandiya training centre.
They were accompanied by far-right national security minister Itamar Ben-Gvir, who echoed long-standing criticism of the agency. The Israeli government claims that UNRWA had become "riddled" with Hamas members, some of whom took part in the October 7 attacks.
Ben-Gvir was filmed sitting behind a desk he said was used previously by the training centre director. "UNRWA is out," he said. "The terrorists must be expelled from every place in the State of Israel."
UNRWA terminated the contracts of nine members of staff accused of involvement in the October 7 attacks, but said Israel has not supplied evidence to substantiate its allegations of widespread infiltration by Hamas.
In a statement, UNRWA said that "it unequivocally condemns" Israel's seizure of the facility and said that "the entry of armed security forces into a United Nations facility, without the authorisation or consent of the United Nations, is unacceptable".
A spokesperson for the UN Secretary-General Antonio Guterres said that Israel's actions were "wholly unacceptable and inconsistent with Israel's obligations under international law".
The centre was established more than 70 years ago and has taught professional skills to thousands of Palestinians.
Separately, the UN's Special Co-ordinator for the Middle East Peace Process, Dr Ramiz Alakbarov, said that the centre should be "inviolable and immune from any form of interference". He called on the Israeli government to "withdraw from the facility" and to observe "its obligations under international law".
Under a 1946 UN convention, UN premises are considered "inviolable" while UN property and assets are immune from "search, requisition, confiscation, expropriation and any other form of interference".
There is no obvious precedent in UN history for a UN member state to take over a functioning UN facility, against the wishes of the UN, to convert the site for its own purposes.
In January, Israeli authorities demolished UNRWA's headquarters compound in Sheikh Jarrah, in occupied East Jerusalem.
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The UN insists the site is still a UN premises, but Israel has already put forward plans to turn it into a military complex, including an army museum, a recruitment centre and an office for the defence mInister.
The Jerusalem municipal authorities claim that the site was never legally owned by UNRWA. They say it will now be redeveloped as an educational and community complex for local residents, including a school, a mosque, health centres and sports facilities.
Oren Marmorstein, spokesperson for the Israeli ministry of foreign affairs, said: "UNRWA has no property rights in this facility and has tried to obstruct the opening of this school. At the end of the day, UNRWA and the UN are choosing to oppose this project. They are choosing politics over children."
Health officials said on Tuesday that the two Pennsylvania residents were both unvaccinated residents of Lancaster County.
Pennsylvania has been one of the hardest-hit states this year, with 393 measles cases across 28 counties so far.
Lancaster County, in the south-eastern part of the state, has had 185 cases, but these were the first measles deaths recorded in the state in 35 years.
So far this year, the US has had 2,777 confirmed measles cases, a disease that had been largely eradicated in the country.
Among the factors contributing to the increase in infections is the declining vaccination rates.
Vaccination rates of 95% or higher can prevent outbreaks by creating what is known as herd immunity that protects people unable to be vaccinated.
Two doses of the Measles-Mumps-Rubella shot provide 97% protection against the virus.
Children typically first receive the vaccine when they are 12 to 15 months old and again at age four to six years of age.
According to Pennsylvania Department of Health data for the 2025-2026 school year, 93.2% of Pennsylvania kindergarten-age children had received the required two doses of MMR vaccine, down from 93.7% during the 2024-2025 school year.
In Lancaster County, 87.6% of kindergarten students received the required two MMR doses during the 2025-2026 school year.
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The number of US measles cases in 2026 is the highest in a single year since the major resurgence between 1989 and 1991, when 55,000 infections and 123 deaths were recorded.
"Because measles was largely eliminated in the Commonwealth for more than three decades, people are not familiar with this disease and don't fully understand the potential severity of the illness," said Pennsylvania Secretary of Health Debra Bogen.
Health Secretary Robert Kennedy Jr and President Donald Trump have in the past promoted a long-debunked link between vaccines and autism, contrary to established scientific evidence.
Earlier this month, Mr Trump issued an executive order calling for a reduction in recommended childhood vaccines.