British Transport Police said they have received reports of multiple casualties.
March 22: Five people are killed and more than 50 are wounded when a man rams his auto into pedestrians on Westminster bridge in London before crashing into the barriers surrounding parliament and then stabbing a police officer to death.
Also Saturday, two men entered a restaurant at Borough Market, just south of the Bridge, and stabbed two people inside, a witness told CNN on Saturday.
In March, four people were killed in London after Khalid Masood rammed his auto into pedestrians on Westminster Bridge, before fatally stabbing a policeman outside the Houses of Parliament.
She said she saw cops arresting someone. Police also say they are responding to incidents at two other locations nearby.
Sky News Home Affairs Editor Mark White said: "Certainly reports we're getting suggest that this may be terror related but that is not being confirmed by police at this stage".
Britain's Sun newspaper said seven people were feared killed and that two attackers were shot dead by police near London Bridge; but there was no immediate confirmation of this.
Reuters reported a witness saying she had seen people with "apparent knife wounds" on London Bridge. Members of the public were told by police to "run as fast as they could" westbound.
On May 22, a British-born suicide bomber killed 22 people and injured 59 others at Ariana Grande's concert in Manchester, England.
"After that I went towards Vauxhall and found Holly and Mary who very kindly gave me a place to stay".
Trains are now not stopping at London Bridge or Borough stations.
He told the station: "I didn't see the van mount the kerb but I saw everything else".
The London Ambulance Service said it had multiple resources on the bridge.
MARTIN: And, Frank, we only have a little bit less than a minute left, but this is all coming just a month after three people were killed near Parliament there in London when a vehicle ran over victims.
Prime Minister Theresa May said she was being updated by officials on the "fast-moving" situation, adding: "I can confirm that the awful incident in London is being treated as a potential act of terrorism". "The Uber stopped, we asked people what was going on - people said there was shooting", said Yoann Belmere, 40, a French banker living in London.
Soon after, reports came in of stabbings around Borough Market, just steps away from the bridge. But they said later that turned out to be an unrelated stabbing.
London Ambulance Service took at least 20 people to six hospitals, the Associated Press reported.
"I thought at first it was fireworks then I recognised it was significant gunfire and that was the first instance", he said.
Marlins' Volquez throwing no-hitter through 7 versus Arizona
This was the fourth time the Yankees have hit four homers in an inning - they last did it October 1, 2012, against Boston. The 33-year-old threw the Major League Baseball season's first no-hitter at the Arizona Diamondbacks in a 3-0 victory.