Witnesses report a vehicle hitting pedestrians and injured people on the ground. Earlier reports of a third incident at Vauxhall about four kilometres away have been found to be unrelated.
Several witnesses said some victims also appeared to have stab wounds.
President Donald Trump argued in favor of his controversial travel ban as London authorities responded to reports of a string of attacks Saturday night. Police say at least one person has been killed.
The suspects are still at large and were reportedly moving on foot to the Borough Market area after fleeing London Bridge.
London police report "incident" on London Bridge.
Scotland Yard have confirmed a stabbing in Vauxhall is not connected.
Police boats were searching the river - apparently looking for people who may have been thrown off the bridge.
The Prime Minister will chair a meeting of the Government's Cobra emergency committee on Sunday, No 10 said.
"Officers jumped out and started tending to the guy on the floor and about three of them also started running towards the London Bridge area".
The US embassy in London tweeted: "Please avoid the area and monitor local news/@metpoliceuk for updates".
London Bridge Tube station was closed and London Bridge was closed in both directions.
A witness told CNN two men entered a restaurant at Borough Market, just south of London Bridge, and stabbed two people inside.
Will Haven, managing editor of The Spectator in London, told Sky he "certainly saw people who looked like they'd been run over".
Casualties were seen being taken away from Tito's restaurant on London Bridge Street with blood on the steps of the eatery.
In March, a man driving an SUV mowed down pedestrians on Westminster Bridge, killing four people, before getting out of his vehicle and stabbing a police officer outside the Houses of Parliament.
On social media, London Met have told people to people to "run - to a place of safety".