A GIRL aged 13 has been arrested on suspicion of attacking a cop at a pro-Brexit "yellow vest" protest in London.

Up to 100 demonstrators clashed with cops in the capital today and set off smoke bombs outside Downing Street during the protest.

The teenager held by cops was one of four arrested on Westminster Bridge today as traffic was brought to a standstill.

Protesters marched across the country calling for a no-deal Brexit as they branded Theresa May a "traitor".

In London, agitators wearing yellow hi-vis vests stood in front of cars and stormed down Whitehall towards Trafalgar Square in London.

Transport for London warned of delays in the area as the Brexiteers sat in the road and blocked traffic.

They said: "The crowd has formed up in the Parliament Square / Whitehall area and are intermittently causing traffic holds.

"We have very little details on the demonstration but expect further traffic holds and delays in the Westminster area."

They gathered by the Houses of Parliament at noon before occupying Westminster Bridge.

As the march gathered speed, protesters chanted "Brexit now" and blocked cars from crossing Tower Bridge and then Waterloo Bridge as the PM holds crunch talks in Brussels.

They were eventually moved on by cops before they headed to Downing Street singing pro-Brexit songs.

Some were seen rowing with motorists and bystanders in London as tensions spilled over this afternoon.

Marches also took place in Manchester as protesters in the yellow vests spilled out into the city centre.

A spokesman for Metropolitan Police said: "Four arrests have been made following protests on Westminster Bridge, SW1 at approximately 11am today.

"Three men arrested on suspicion of public order offences. A 13-year-old girl was also arrested on suspicion of assault on police. Enquiries continue."

The Yellow Vest movement started in France as a protest over fuel tax rises but have escalated into expressions of wider discontent about President Emmanuel Macron’s policies.

Around 136,000 protesters wearing the "gilets jaunes" took part in nationwide demonstrations last month.

Violence has since escalated after the dramatic arrest of one of their leaders, Eric Drouet as he tried to lay flowers and light candles in memory of ten people killed during demonstrations.