Thousands of protesters gathered near the prime minister's residence at 10 Downing Street in central London. Other events were held in Belfast, York and elsewhere in the country.
The demonstrators wanted to express their determination to block an exit from the European Union "without agreement".
The opposition Brexit movement was galvanized by Boris Johnson's decision to close Parliament for several weeks to prevent a new debate on the subject.
In London, protesters chanted "Boris Johnson, shame on you". Some carried posters "Stop the coup". According to them, the decision of the new prime minister undermines their rights and threatens democracy.
These events were organized by an anti-Brexit group, "Another Europe is possible", and by "Momentum", an organization linked to the Labor Party. He urged his members to "occupy the bridges and block the roads".
According to the organizations, events were to be held in more than 30 cities in England, Scotland, Northern Ireland and Wales.
In Exeter, in the west of the country, pharmacist Bridie Walton was demonstrating for the first time in her life.
Nobody voted for the establishment of a dictatorship, she commented. It is the decision of a man who fears that his arguments will not hold water if they are examined.
Some MEPs want to introduce a bill to prevent a UK exit from the European Union without an agreement. Their plan will become more difficult to achieve if Parliament is closed for part of the pre-Brexit period on October 31.
The suspension of Parliament is also the subject of three court challenges, which will be discussed in court next week.