Downing Street was on partial lockdown as protesters gathered outside the gates and shouted objections to Boris Johnson becoming prime minister.
Crowds swarmed onto Whitehall, waved signs and let off smoke bombs after they marched through central London.
A large police presence gathered at the end of Downing Street as red smoke poured into the air.
MP Grant Shapps turned up at the gates of Downing Street just before 8pm but was told by an armed officer that he would not be able to use the main entrance.
Security officers at the gates said the site had been put on lockdown after the protesters arrived, with no one being allowed in or out of that entrance.
A helicopter buzzed overhead as the protesters, who were carrying placards which read "Boris is a no no" shouted their disapproval at the new prime minister.
On the other side of the gates, away from the demonstration, a string of Conservative MPs made their way out of No 10 as further appointments were announced.
They each walked straight from No 10 and to a private exit which took them towards the Foreign Office buildings or a back exit and none left by the front gates.
Protesters also gathered around Parliament throughout the evening, after originally congregating in Russell Square.
A spokesman for Scotland Yard said the demonstration was “noisy but good natured”, with one arrest related to drug offences as people marched through the capital.
Shadow Chancellor John McDonnell spoke from a bus which organisers coordinated to travel along the route earlier in the day.
An event page organised before the march said: “We will make ourselves heard - we will not let this man become PM without a fight.”
