London police were on the scene Saturday night of what transit officials called a "major incident" on the London Bridge.
NBC News' British partner ITV News reported that people in the area said a van struck multiple pedestrians on the bridge.
According to the Associated Press, British transport police say there are reports of multiple casualties.
London's Metropolitan Police Service said on Twitter just before 10:30 p.m. local time (5:30 p.m. ET) that it was dealing with an incident on the iconic bridge. Police later said that it was also responding to reports of stabbings at Borough Market, and armed police are at the scenes. Police said shots have been fired.
Police are also responding to a third incident in the Vauxhall area of London.
The London Ambulance Service said it had multiple resources on the bridge.
Videos on social media showed people being evacuated from the area. Witness Peter Joshuas told NBC News he was just exiting the bridge when he heard screams and people shouting, and people running in confusion.
London police cautioned on Twitter: "We are aware of reports on social media. We will release facts when we can — our info must be accurate." Police have not characterized either incident as being believed to be connected to terrorism.
White House Press Secretary Sean Spicer said on Twitter that President Donald Trump has been briefed on the situation.
NBC News producer Mo Abbas was at a restaurant near the south entrance to London Bridge, when the incident happened. He said a panicked store manager told diners to abandon their meals and shelter in the back of the restaurant while police conducted a search of the area.
After about 10 minutes, Abbas said diners were escorted single-file out of the restaurant and led away from the area by armed police in body armor.
The incident was not immediately known to be terrorist-related, but it follows a March attack in which a man plowed a vehicle into pedestrians on the Westminster Bridge and fatally stabbed a police officer in what authorities said was a terror attack.