A young man has died after being stabbed in a building housing Brent Council's headquarters and a library.
Emergency services were called at about 14:30 BST to Brent Civic Centre, near Wembley Stadium in north-west London, where the victim was pronounced dead at the scene.
The Met Police said he was believed to be a teenage boy or man in his early 20s.
Councillor Muhammed Butt, leader of Brent Council, said: "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. I would urge people to avoid the area and allow the police and emergency services to carry out their work.
"There is an active police investigation under way - please do not speculate or repeat any unverified claims online.
"We are working closely with the Metropolitan Police and will provide further updates as soon as we are able."
'Stop-and-search powers'
Supt Chris Clarke, the Met's lead for emergency response policing in the area, said: "This is an awful incident, in which someone has tragically lost their life. We are working to urgently identify the victim and inform his next of kin as soon as possible.
"We recognise the concern this incident will cause within the local community and are committed to keeping residents informed as our enquiries 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.
"There will be an increased police presence in and around the area in the coming hours while our officers continue their investigation."
A London Ambulance spokesperson said: "We were called at 14:25 BST today to reports of a stabbing at Brent Civic Centre, Wembley.
"We sent an ambulance crew, paramedics in fast-response cars and an incident response officer. We also dispatched London's Air Ambulance.
"Sadly, despite the best efforts of our crews, a patient was pronounced dead at the scene."