One witness told The Independent: 'I heard police shouting – at first I thought they were just shouting at tourists to stay back – but there was a guy stood just inside the gates facing [away from Parliament]. Journalists tweeting from the scene described a heavy police presence and a man being “pinned to the ground” inside Carriage Gates, where an attacker in 2017 broke into Parliament before killing P Keith Palmer. Parliament is in lockdown and there is a heightened police presence on the estate, after an alleged intruder was reportedly tasered by armed police within the estate by Carriage Gates, having made it a few meters inside. UPDATE II: The intruder has now been taken away in a police van, and the Metropolitan Police have released this statement:

“A man was detained and arrested by Carriage Gates inside the Palace of Westminster on suspicion of trespassing at a protected site, at around 11.55hrs on Tuesday, 11 December. I heard a crack, which I guess was the Taser because I saw the Taser wires after, and he went down and very quickly they were all on him.' Enquiries into the circumstances continue.”

While the Palace of Westminster — which is home to both houses of the UK Parliament — has been the target of several terror attacks in the past and is the scene of an ongoing anti-Brexit demonstration, there has been no suggestion the arrested intruder Tuesday is related to either of those ongoing security concerns.
In the United Kingdom, a man is tasered by police at the gates of the Palace of Westminster. The area is placed on lockdown.