Few lawmakers have any appetite for a second public vote, however, so the march’s main impact may be as a reflection of growing popular frustration over the gridlock.

What’s next? Officials in the E.U. gave the British Parliament until April 12 to decide what it wants to do, so Brexit will not take effect on March 29, the date set two years ago.

On the table is Prime Minister Theresa May’s widely unpopular draft plan. If lawmakers approve it by the E.U. deadline, Britain will leave the bloc on May 22.

If not, Britain has three choices: a no-deal Brexit on April 12, no Brexit at all or a longer delay of possibly two more years. Parliament has strongly opposed all of these options, too.

Today: Lawmakers are to vote on proposed amendments to Mrs. May’s plan, including one that would allow for a rapid-fire round of voting on alternatives.

Analysis: Ellen Barry, our London correspondent, looks at how some Brexit supporters have shifted from the original goal of a less restrictive relationship with the E.U. to willingness to walk away from the bloc with no deal at all.