A man has been shot dead by armed officers in London Bridge after he was reportedly seen carrying a knife.
It is believed somebody was stabbed in the area and police have shot a suspect, a security source said.
Video footage appears to show a member of the public wrestling a man on the ground before police intervene.
Witnesses reported hearing up to 10 gunshots as 'sirens wailed' and officers evacuated London Bridge station and the surrounding area.
One witness claimed there was "a man running around with a knife and been shot in London bridge outside my office just witnessed with my own eyes. (sic)"
Another witness said: "I'm about a 3/4 mile from London Bridge and could hear the gunshots - lots of them.
"Seem to have stopped. Sirens wailing now. Hope people are ok."
Dan Castleton wrote on Twitter: "I heard at least 10 shots fired and a couple of bangs like flash bangs serious incident."
A cordon has been set up at Borough Market while evacuation continues from nearby shops and businesses.
Videos and photographs on Twitter showed several police cars and buses on the bridge and a truck straddling several lanes.
London ambulance service said it had crews at the scene.
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Mirror Online journalist Talia Shadwell was among the crowds caught up in the panic.
She said: "We were just in Marks & Spencer near Eastcheap and then there was policing shouting and telling people to back off and then more and more people were started running.
"There's a lot of vehicles filing past, police vans.
"My friend saw people on their phones crying.
"Police were shouting at people to move out the street, people were running behind us and so we started running as well.
"It's not often you hear police being that forceful."
Timothy Johnson tweeted: "Police shouting for everyone to move away. Real panic setting in at London Bridge.
"Rumours of a shooting?"
Another said: "Security incident at London Bridge. Armed police. Gunshots fired."
"Gunshots heard on London Bridge from our office," another added.
Detectives have said they are in the early stages of dealing with an incident.
They added: "If you are near the scene, please follow the directions of any officer on the ground."
The area, one of the busiest in the capital, has been cordoned off by police.
London Bridge was the scene of an attack in June 2017 when three militants drove a van into pedestrians and then attacked people in the surrounding area, killing eight people.
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