The pro-Brexit demonstrators blocked streets around the Houses of Parliament in central London on Friday as several groups of protesters converged on the day that the UK was originally meant to depart from the 28-member bloc.
Marchers carried placards that read, "Every nation has the right to self-determination," and "Leave means Leave."
The demonstrators also chanted slogans, including "Brexit now!" and "we want our Brexit back."
Protesters against May’s Brexit deal stood outside parliament, wearing a blue top hat adorned with a yellow ribbon reading "Stop Brexit."
The rallies were held after British lawmakers voted 344-286 against May's 585-page EU Withdrawal Agreement for a third time.
The British premier had told parliament that the vote would be the last opportunity to ensure Brexit would take place, also warning that if the deal failed, then any further delay to Brexit would probably be a long one beyond April 12.
The European Council President Donald Tusk responded to the outcome of the vote, saying, "In view of the rejection of the Withdrawal Agreement by the House of Commons, I have decided to call a European Council on 10 April."
The Labour Party, which opposes May's Conservative Party, said in a tweet that the British government was "in chaos" after the Friday vote.
British people voted to leave the EU in a 2016 referendum.