A man is reported to have thrown petrol over himself outside the Houses of Parliament.
A huge police response was triggered after the unidentified person doused themselves in liquid, MP Huw Merriman tweeted.
Our reporter at the scene said police held a person up against a wall next to Carriage Gates in the moments after the dousing.
Officers were seen spraying a fire extinguisher on an unknown substance before wrapping the man in a blanket.
Carriage Gates is the main entrance to Parliament off Parliament Square and it is where protesters gather.
It is also where PC Keith Palmer was killed in the 2017 Westminster terror attack.
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An eye witness claimed the suspect doused himself in petrol from a two litre coke bottle and police at Parliament rushed to the scene to pin him down while someone raced for a fire etinguisher.
A BMW X5 police armed response vehicle stationed nearby raced to the scene while armed officers guarding Parliament hurried to deal with the suspect.
A fire engine has now arrived to help clean up the spilt petrol.
Police have closed part of parliament square.
A paramedic rapid response vehicle and an ambulance also rushed to the scene.
The suspect, wearing a red anorak, was lifted to his feet and pinned against a wall and black iron railings running around Parliament.
A police cordon of some twenty yards was set up as heavy rain fell on Wetminster.
A man who looked to be in his 20s, wearing jogging bottoms sweat shirt and trainers, was led by police and paramedics to an ambulance.
They put his hands on the back of his head.
He is being searched for weapons.
The Metropolitan Police tweeted: "A man was detained in #Parliament Square #Westminster at approx
10:45hrs today after dousing himself with what appeared to be a flammable liquid.
" No injuries. LAS are on scene and he is currently being assessed."
