A man has been arrested at Whitehall on terror offences after he was found with knives near London's Houses of Parliament.

Armed officers were seen on Whitehall, the street home to numerous government ministries in central London at 2.20pm (11.20pm AEDT).

The suspect was arrested after allegedly being found with a number of knives. (AFP)

The suspect was held on the ground near London's Houses of Parliament. (AFP)

Police said no one had been injured in the incident and the man, who is in his late 20s, was "arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism."

Footage of the scene posted on social media showed the man lying on the ground with heavily armed officers around him, before he was place in an unmarked police car and driven to a south London police station.

An AFP photographer saw firearms officers surrounding the man, who was dressed in dark clothing, and pinning him to the ground at a pedestrian island.

The bearded man, with close-cropped hair, was seen being detained closed to a telephone booth used by countless tourists posing for photographs with the Houses of Parliament in the background.

David Wisniowski, who was working on a building site right next to the incident, told AFP he could see "three knives on the floor, one big one and two small".

Forensic officers look at the knives found at the scene. (AFP)

Gabrielle Hennessy, 20, a US tourist, described seeing police rush at a man.

"We were walking along Parliament Square and we saw police rush at him. And then he was on the ground surrounded by police. They stood him up and that's when I saw the knife on the ground," she said.

They added the arrest occurred "following a stop and search as part of an ongoing operation."

The foiled plot comes less than a week after the Westminster attack. (AFP)

One man has been arrested for an alleged terror plot. (Twitter)

A BBC Home Affairs correspondent said on Twitter that two knives had been seen on the ground including a large "breadknife".

British police closed the main gates to parliament a Reuters witness said and buses around the area were being diverted.

"Detectives from the Counter Terrorism Command are continuing their investigation," police said in a statement.

"As a result of this arrest there is no immediate known threat."

The incident comes just a month after Khalid Masood drove a rental car into unsuspecting pedestrians, killing American tourist Kurt Cochran, 54, retired window cleaner Leslie Rhodes, 75, and Aysha Frade, 44, before fatally stabbing PC Keith Palmer, 48, in the Palace of Westminster's forecourt.

The incident happened close to Prime Minister Theresa May's Downing Street office.

Ms May was outside London on a general election campaign visit at the time, a spokeswoman said.

"This is an ongoing police operation and we are monitoring it," she said.

Police said no one was injured in the incident. (Twitter)

"The prime minister has been informed by officials and she will be updated, as usual for incidents like this."