Bomb squad experts were seen arriving at the scene this afternoon as Londoners and tourists were quickly moved to the side.
While a bomb robot was seen being removed from the bomb squad van underneath the rail bridge leading to the bus station.
The bridge was closed to both drivers and pedestrians now, and both the train and bus stations have been evacuated.
Building in the area, including the Shard were also reportedly being evacuated – but police have now confirmed the the incident was not suspicious, and the roads around London Bridge are now being re-opened.
Pictures released as bomb squad enter London Bridge station Tue, March 7, 2017
Bomb squad experts were seen arriving at the scene as Londoners and tourists were quickly moved to the side
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Witnesses claim the dramatic scenes that unfolded in the busy area saw lots of police, paramedics and fire services at the scene, with more police arriving all the time.
The sounds of sirens filled the air as dozens of police vans continued to surround the birdge, along with motorbikes, cars, bicycles and fire engines.
Sniffer dogs were also deployed to the northern side of the bridge just outside Monument station, and were spotted pacing the area outside Waitrose and Pret a Manger.
London Bridge was closed off at about 1.20pm
The bridge has now been completely cleared of people
A PCSO at the scene said there was a "suspicious package" on the bridge at the London Bridge station edge.
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And other officers at the scene reported the bridge is likely to be closed for two to three hours – while witnesses in the area claimed a suspicious vehicle had been discovered close to the busy London station.
But the Met police were unable to confirm this, describing the incident only as "a security alert".
Police vehicles are queueing up at Monument station across the bridge
Commuters are being forced to wait outside London Bridge station
Express reporter Vickiie Oliphant, at the scene, said: "There are lots of police, with several squad cars and ambulances arriving.
"The roads are very chaotic around the junction by Monument station as the police have taped off the bridge and are evacuating pedestrians.
"There are rumours it is a bomb.
"They're bringing in truckloads of officers and there are queues of people lining up at the police cordon."
Swarms of police, fire engines and ambulances were on both sides of the bridge
One officer said a suspicious vehicle was found at the scene
While hordes of workers stuck outside the station or near the bridge have taken to social media to live tweet the chaos.
Journalist Anila Dhami tweeted: "#LondonBridge station evacuated. Loads of security services including police and fire. Large area cordoned off with more police arriving."
Lizzie Pook said: "Just been evacuated from #LondonGrind in #London bridge. Being shouted at a lot by police to hurry up across the bridge."
Paul Vanston wrote: "Best avoid area: Full security alert for #LondonBridge
"King William St sealed off to traffic. Bridge closed to pedestrians #Blackfriars open."
Ben Monteith, who has been evacuated from his office near London Bridge station to Monument Underground station, said: "People don't really understand what is going on. There is defintely some concern and the police are just telling people to go home.
"They have closed off London Bridge and pushed back to Monument."
All trains were halted at London Bridge station
A Met Police spokesman said: “We were called to a security alert in London Bridge street at 1.40pm. The incident is ongoing.”
A spokesman for BTP said: "The station has been closed while Metropolitan Police investigate. The incident is ongoing outside the station.
"We have officers assisting the Met."
Disruption to Southeastern services are expected until 4pm, the beleaguered train company said.
Network Rail tweeted: "The London Bridge Bus Station has been evacuated."
Pedestrians were stopped from crossing the bridge
A bomb robot was called in by police
A spokesman for the rail company has given issued travel advice to passengers who had planned to travel to London Bridge.
He said: "The station has been closed while emergency services deal with an incident nearby.
"No trains are running to or through it, which means that there are no services running from Charing Cross, Waterloo East and Cannon Street.
"Passengers should check with their operator before they travel this afternoon as significant disruption is expected."
The station has been cleared and all surrounding building
Police have also requested an exclusion zone, so no trains will run towards London Cannon Street or London Charing Cross until National Rail are told it is safe to do so.
Southeastern will be diverting trains to London Blackfriars and London Victoria where they can, but some trains will need to be terminated before reaching London.
Some Southern services will start and terminate at New Cross Gate while others services will be revised or diverted on route.
Southeastern Railway tweeted: "Because of a security alert at London Bridge, the station has had to be evacuated and at present no trains can pass through. More to follow."
The incident comes just as the President of Rwanda was supposed to arrive at a building just off London Bridge, where a protest against his welcome was due to take place.
Paul Kagame is believed to be attaending a Wall Street Journal talk today on their Investing in Africa campaign.