At least 20 casualties have been taken to six hospitals following terror incidents at London Bridge and Borough Market.
Emergency services are responding to reports that a van ploughed into pedestrians on London Bridge and of stabbings at the popular food market in south London.
London Ambulance Service said: "We can confirm we have taken at least 20 patients to six hospitals across London following the incident at London Bridge.
"We have also treated a number of people at the scene for less serious injuries.
"We have declared a major incident and continue to work closely with other members of the emergency services.
"Our priority is to ensure patients receive the medical help they need as quickly as possible."
Emergency services were seen treating people lying injured at the junction of Thrale Street and Southwark Street, near Borough Market.
British Transport Police said there were reports of "multiple" casualties but that the seriousness of the injuries was not yet known.
An image shared by a photographer on social media showed a man on the ground with what appeared to be grey canisters around his waist by the Wheatsheaf pub in Borough.
He told the BBC he saw three men shot by police.
Explosions, believed to be controlled, have since been heard in the London Bridge area.
A third incident involving a stabbing at Vauxhall was unrelated, the force added.
Prime Minister Theresa May said she was being updated by officials on the "fast-moving" situation and thanked the emergency services, adding: "Our thoughts are with those who are caught up in these dreadful events."
She will chair a meeting of the Government's Cobra emergency committee on Sunday, No 10 said.
Scotland Yard said officers were responding to three incidents in the capital.
:: Police were called at around 10.08pm to reports of a vehicle hitting pedestrians on London Bridge.
:: Officers then attended a second incident at nearby Borough Market after reports of stabbings. Armed officers responded and shots were fired.
:: There are believed to be multiple casualties.
:: An explosion was heard from the London Bridge area at around 1.50am
:: Nearby Guy's and St Thomas' NHS Foundation hospital said it was "on lockdown to keep patients, relatives and staff safe".
:: The third, unrelated, incident was at Vauxhall, Scotland Yard said.
Witnesses described seeing a speeding vehicle veer into crowds on London Bridge.
A man called Gerard told BBC news how witnessed a series of stabbings by three men near Borough Market.
Meanwhile video showed dozens of people with their hands on their heads escorted away from the affected areas by officers.
"They were stabbing everyone. They were running up and going 'This is for Allah'," Gerard said.
He said he followed the attackers and threw items at them in a bid to stop them.
Alex Shellum, who was in The Mudlark pub, close to London Bridge, said he saw a bleeding woman in her early 20s stagger in.
"It appeared to myself and to my friends that her throat had been cut," he added.
Barman Alex Martinez was forced to take shelter in a bin when a knifeman stormed El Pastor, the Borough restaurant in which he worked.
He said: "I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening.
"The man started screaming and dropping everything on the floor, the plates, the cutlery, the bottle, everything, he dropped it."
He said he jumped down behind a wall to keep safe.
Emergency services were seen taking one woman away on a stretcher while others sat injured on the ground, with shocked people crying and shouting around them.
Frightened people were led up Southwark Street away from Southwark Bridge Road, and police officers were seen screaming to people and ordering them to run.
Many were in tears, with friends supporting each other and carrying people up the road.
Cordons were put up across Southwark Street before repeatedly being pushed back as police widened the critical zone around Borough Market.
London Bridge was closed both ways TfL said, warning the public to avoid the area.
BTP confirmed London Bridge station would remain closed throughout the night.
London Ambulance said it had sent a number of ambulance crews, advanced paramedics and specialist response teams to the scene. London Fire Brigade also attended.
Police boats were searching the river – apparently looking for people who may have been thrown off the bridge.
People staying in the Premier Inn Bankside were evacuated across Southwark Bridge to a nearby hotel.
Dozens, including children, were brought across the bridge by police after the fire alarm in the hotel went off, according to one.
Samuel Templeton, from Belfast, one of the guests in the hotel, said there was a heavy police presence and they were told to cross the bridge in single file.
St Thomas' said it was one of the six hospitals treating patients.