A man has been tasered and arrested inside London's Palace of Westminster.

The Met Police confirmed a man had been taken into custody after being arrested by Carriage Gates inside Parliament about 11.55am on Tuesday (10.55pm AEST).

"Stop. Get down," police shouted, a Reuters photographer who witnessed the incident said.

Pictures showed armed police using a stun gun on the man on the cobbled forecourt inside the British parliament's main Carriage Gates.

The man was later handcuffed as police pointed a gun at the man's head.

"I heard a scream and then looked out the window, and then about half a dozen police officers who appeared not to be armed were running towards a man," Daily Telegraph UK reporter Christopher Hope told the BBC.

"And then it appears the man was tasered.

"It appears to be contained now.

"There are armed police patrolling downstairs by the Christmas tree."

The man was arrested on suspicion of trespassing at a protected site, Met Police said in a brief statement on Twitter.

"Enquiries into the circumstances continue."

The House of Commons said on Twitter that the incident was now over and was being handled by the police.

In March 2017, Khalid Masood killed four people on nearby Westminster Bridge and stabbed an unarmed police officer to death near Carriage Gates before being shot dead.

In August, a man was arrested after driving a car into pedestrians and cyclists before ramming into barriers outside the parliament building in what police said appeared to be a terrorism attack.