LONDON — Lawmakers voted to withhold support for Prime Minister Boris Johnson’s new Brexit deal, scuppering his hope of finalizing Britain’s exit plan at an extraordinary “Super Saturday” session in Parliament.

The humiliating defeat, however, did not deal a fatal blow to the withdrawal agreement he negotiated in Brussels. Johnson said he planned to press ahead and seek approval of his Brexit deal in the coming week.

Britain remains deeply divided over Brexit. On Saturday, organizers estimated that protests pushing for a second referendum drew 1 million people into the streets of London in on-and-off rain.

Johnson is relying in part on “Brexhaustion” to get his deal passed. Number crunchers said the outcome would be too tight to call, but there were ways he could secure a majority.

Saturday’s successful amendment, from Conservative Party rebel Oliver Letwin, was designed to box in Johnson — so he cannot force Britain to leave the European Union until lawmakers have scrutinized and passed all necessary legislation for an orderly exit.

Johnson’s allies branded it a sneaky attempt by an obstreperous Parliament to defy the will of the people and gum up Britain’s exit trajectory.

The vote was close: 322 in favor and 306 against.

It was an anticlimactic conclusion to a day when lawmakers gathered on a Saturday for the first time in 37 years, since Britain fought in the Falklands.

Johnson responded to the parliamentary beatdown with emphatic finger-jabbing. The prime minister insisted, “I’m not daunted or dismayed by this particular result.” He vowed that he would “not negotiate” a delay with the E.U. — which didn’t mean he wouldn’t ask for one.

As it turned out, the president of the European Council, Donald Tusk, announced late Saturday that the British government had notified the E.U. of its extension request.

“I will now start consulting E.U. leaders on how to react,” Tusk said. 

 Johnson had warned the House of Commons that “further delay would be bad for this country, bad for our European Union and bad for democracy.”

According to legislation passed last month, if a deal was not approved by 11 p.m. Oct. 19, Johnson was required to formally seek a three-month extension beyond the Oct. 31 deadline.

In an unsigned letter sent by the British prime minister to Tusk, the government requested a Brexit delay until the end of January 2020. The government said that if it managed to ratify the withdrawal agreement sooner, the extension could be shortened. 

 European leaders are sick and tired of Brexit talks, but are almost certain to agree to a short reprieve.

“If Johnson asks the European Union to grant an extension, it should be approved, since a modified agreement on the terms of withdrawal has been reached,” Latvian Foreign Minister Edgars Rinkevics told his country’s LETA news agency. “It is in everyone’s interest that the Brexit is arranged in an orderly manner.”

The scene as London Parliament debates Brexit in a historic session

Oct. 19, 2019 | E.U. supporters march to demand a second Brexit vote while lawmakers gather in Westminster Palace on “Super Saturday” for a rare sitting of the House of Commons. (Henry Nicholls/Reuters)

After the action at Westminster, lawmakers on both sides of the Brexit divide required police escorts to get past the hostile demonstrators outside.

“Why do the so called ‘People’s Vote’ protesters think it’s ok to abuse, intimidate and scream in the face of someone they don’t agree with,” tweeted cabinet member Andrea Leadsom. “So frightening, and so grateful to the police.”

The demonstrations, though, were largely peaceful.

Lawyer Saira Ramadan, 36, was there with her husband and two children. She said it was “our last real opportunity to make our voices heard as publicly as possible, and in large numbers.”

Asked about “Brexhaustion” and the claim that Britons just want Brexit done, she said, “It would be disingenuous of me to suggest that there isn’t a feeling of Brexit fatigue . . . but that’s not to say that should be a reason for those of us who feel strongly enough to take it lying down and give up because we want it done.”

For more than a year, polls have shown that if there were a countrywide “do-over vote,” Britons would, by a narrow margin, opt to stay in the E.U.

Polling firm YouGov reported that 30 percent of Britons favor Johnson’s deal, 17 percent want to get out without a deal and 38 percent want to remain, with the final 15 percent unsure.

Johnson on Saturday swatted away calls for a second referendum and continued to press lawmakers to get Britain out of the E.U. by the end of October, as he has promised many times, “do or die.”

In his remarks to the House of Commons, he emphasized the toll of the Brexit debate — which he launched as a leader of the 2016 referendum campaign.

“Friendships have been strained, families divided and the attention of this House consumed by a single issue that has at times felt incapable of resolution,” he said.

The prime minister called his deal “a new and better way forward” for Britain and Europe.

The leader of the opposition Labour Party, Jeremy Corbyn, said lawmakers should reject it. 

“I also totally understand the frustration and the fatigue across the country and in this House,” Corbyn said. “But we simply can’t vote for a deal that is even worse than the one the House voted to reject three times.”

Even some friendly lawmakers who support Brexit complained that they wanted time to at least read the government’s economic analysis of Johnson’s deal before they vote on it.

“His strategy has been the same as Theresa May’s strategy,” said Simon Usherwood, a politics professor at the University of Surrey. “Present a deal, and then bounce, bounce, bounce it straight through. Before you know it, you’ve agreed something, and don’t worry about the details.”

As prime minister, May presented her withdrawal agreement to Parliament three times — and three times she was rejected.

On Saturday, May, now a backbencher, rose to speak in the chamber and confessed a sense of deja vu. She, however, offered full-throated support for Johnson. If Parliament doesn’t back the deal, May said, “it is guilty of the most egregious con trick on the British people.” 

Her voice rising with passion, May said, “If you don’t want no deal, you have to vote for a deal. Businesses are crying out for certainty. People want certainty in their lives.”

Johnson’s deal offers a more distant relationship with the E.U. than the agreement she struck. His plan, however, would see Northern Ireland stay largely aligned to the E.U., even though it would leave the bloc with the rest of the country. 

Northern Ireland’s Democratic Unionist Party said the deal was not in the province’s “long-term interests.” The party’s 10 lawmakers are expected to vote against it.

“It was once said that no British prime minister could ever agree to such terms,” Democratic Unionist lawmaker Nigel Dodds said in Parliament. “Will he now abide by that and reconsider the fact that we must leave as one nation together?” 

John Major and Tony Blair said in videos published for the People’s Vote campaign that Johnson’s deal risked derailing peace in Northern Ireland. The two former prime ministers, who both backed “remain” in the Brexit referendum, played important roles in the Good Friday Agreement, the accord that helped to usher in peace in Northern Ireland after decades of sectarian violence.

“It is a shame and an outrage frankly that Northern Ireland is treated like some disposable inconvenience to be bartered away,” Blair said.

Without the support of the Democratic Unionists, Johnson has had to look for other pathways to a majority for his deal. There has been much wheeling and dealing at 10 Downing Street. There was speculation that Johnson might offer the 21 lawmakers he expelled from his party last month a way back in if they voted with him.

He also offered new pledges Friday night to protect workers’ rights, which was seen as an attempt to woo more Labour lawmakers, especially those who are either Brexiteers or who represent Brexit-backing constituencies.

Labour’s Corbyn called those pledges “empty promises.”

This deal, Corbyn said, would “absolutely, inevitably lead to a Trump trade deal, forcing the U.K. to diverge from the highest standards and expose our families once again to chlorine-washed chicken and hormone-treated beef.”

But some Labour lawmakers representing Euroskeptic constituencies have indicated they will support Johnson’s deal.

In an opinion column in the Guardian newspaper, Melanie Onn, a Labour lawmaker from Grimsby, a pro-Brexit town populated by “Labour-leavers,” implored her colleagues “to use this unique chance to help us move on.” 

“The risk of letting this final shot at a deal slip through our fingers is too great,” she said in a piece she wrote with a Conservative Party lawmaker.

Johnson is also hoping to bring on side the 28 hard-line Brexiteers from his party who have previously been resistant to a Brexit deal. That group said Saturday morning that it had advised its members to vote for Johnson’s agreement.

Andrea Jenkyns, a Conservative lawmaker who never once voted for May’s deal, tweeted: “After much consideration, I have decided to back @BorisJohnson deal. Obviously I would prefer No-Deal but I believe we are in real danger of losing Brexit with the Remain shenanigans and the stakes are dangerously high at the moment.”

Michael Birnbaum in Brussels contributed to this report.