Protesters have had to be separated by police during clashes yards from Parliament this evening.
Video footage from the scene appears to show groups of demonstrators pushing against police, who had their batons drawn.
Witnesses said Whitehall had "descended into chaos" as tempers boiled over and red flares were set off.
There were chants of "no to racism" and "no to Johnson" as hundreds of anti-Tory crowds made their feelings known outside Downing Street, shouting they would "never be defeated".
Police outside the Cenotaph called for reinforcements, with flashing blue lights seen on Whitehall, Millbank and Haymarket.
Police were overheard shouting "box them in" as they tried to contain the crowds with a cordon.
Whitehall was briefly closed, and the Met Police told Mirror Online there are demonstrations taking place, but no arrests were believed to have been made.
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Transport for London posted on Twitter : "Whitehall is closed both ways from Trafalgar Square to Parliament Square."
A short time later it said Whitehall had reopened, stating the demonstration had progressed onto Haymarket.
Protesters cried "Save our NHS" as police sealed them in on Victoria Street, and sat on the floor as officers formed a line to block off the scene.
Some people congregated at Whitehall were wearing masks, and police could be seen
Protesters holding banners, including some stating "Boris Johnson not my Prime Minister" were seen bringing traffic to a halt in Haymarket.
Others brandished placards with anti-racist messages on them.
It comes after the Tories won an 80 seat majority after Brits went to the polls on Thursday.
In a miserable 24 hours for Labour, the party lost 59 MPs as voters inflicted the worst result since 1935.
In a victory speech earlier today, the Prime Minister called for Britons to put the deep divisions of the past three years behind them.
In a statement, Mr Johnson said: “This morning I went to Buckingham Palace and I am forming a new Government and people will arrive at Westminster to form a new Parliament.
“I am proud to say that members of our One Nation Government, a people's Government, will set out plans for constituencies who have never returned a Conservative MP for 100 years.
“We will have an overwhelming mandate because of this election to get Brexit done and we will honour that mandate by January 31."
