Demonstrators carried signs with slogans such as: “Brexit: Ugly word, ugly idea.”

Andrew Adonis, a Labour member of the House of Lords, said on Saturday that “voters will neither forgive nor forget” if lawmakers allowed “this miserable Brexit to proceed without people being given the final say.”

Britons voted to leave the trade bloc by a narrow margin in a 2016 referendum. Prime Minister Theresa May has ruled out another public vote on the subject.

Britain is scheduled to leave the bloc on March 29, but negotiations have been plagued by disagreements, particularly over the issue of the border between the Republic of Ireland and Northern Ireland, which will be the United Kingdom’s only land frontier with the European Union after the process known as Brexit.

There are growing fears of a “no deal” exit, which could create chaos at the borders and in the economy.

Mrs. May said at a summit meeting in Brussels this past week that she would consider a longer post-Brexit transition period — one that could keep Britain aligned to European Union rules and obligations for more than two years after its departure. Pro-Brexit politicians in Britain saw it as an attempt to bind the country to the bloc indefinitely.