In an extraordinary reversal at an extraordinary news conference, Rep. Anthony Weiner of New York admitted Monday afternoon that he had repeatedly lied to his constituents and the country in denying that he had sent a lewd picture of himself to a college-age woman on Twitter.

In a tearful admission, he said that he had in fact sent multiple inappropriate messages to multiple women but that he had done nothing illegal and would not resign.

“The picture was of me, and I sent it,” said Weiner (D), who called it “a very dumb thing to do,” “a hugely regrettable mistake” and “destructive.”

“I am deeply ashamed,” said Weiner, his jaw clenched.

Soon after he finished speaking, House Minority Leader Nancy Pelosi (D-Calif.), who he said had urged him to tell the whole truth, called on the House ethics committee to investigate the case.

For Weiner, a seven-term congressman who was seen as a leading candidate to become the next mayor of New York, the collapse is all the more stunning in light of the heights he had reached in his party and as a spokesman for its liberal wing. In the aftermath of his mea culpa, Republican leaders mostly stood back and watched the public self-immolation of one of their sharpest antagonists.

His closest allies limited themselves to offering moral support.

“I am deeply pained and saddened by today’s news,” Weiner’s onetime mentor, Sen. Charles E. Schumer (D-N.Y.), said in a statement. “By fully explaining himself, apologizing to all he hurt and taking full responsibility for his wrongful actions, Anthony did the right thing. He remains a talented and committed public servant, and I pray he and his family can get through these difficult times.”

Speaking in a Midtown Manhattan Sheraton, Weiner, 46, known for his swagger and barbed quips, choked up as he expressed regret to his wife of less than a year, Huma Abedin, a close aide of Secretary of State Hillary Rodham Clinton’s. He then apologized to the media that had transformed the Brooklyn firebrand into a national star.

That humbled self-assessment was in contrast to the bombast that Weiner demonstrated last week, when he sought — to disastrous effect — to shame reporters for asking questions about a photo sent from his Twitter account. He repeatedly said his account had been “hacked,” but he failed to answer basic questions — such as whether the photo was of him and why he hadn’t called on authorities to investigate the matter. All that defiance appeared drained from the lanky congressman Monday as he stood for more than a half an hour taking fire from reporters, who asked whether he had used any government computers to send the messages, whether he knew any of the recipients to be underage, whether he had engaged in phone sex, whether his wife would stand by him and whether he thought he could be reelected.

“I don’t believe I did anything that violates any law or any rule,” said Weiner, who also said he didn’t think he had used any government resources to send the messages, specifying that he used his personal BlackBerry for the interactions. “I don’t see anything I did that violated any rules of the House. I don’t see anything that violated my oath of office to uphold the Constitution.”

To be clear, he added, “I am not resigning.”

Weiner’s admission that he had engaged in inappropriate — but always virtual — relationships with at least six women whom he found on Facebook over the past three years brought the latest act of his remarkably confusing political drama to a close.

Weiner had emerged in recent years as a superstar of liberal-leaning cable TV shows and blogs, excoriating Republicans at every turn. So it was only fitting that before he arrived, Andrew Breitbart — the conservative blogger who first seized on the Twitter photograph and posted newly acquired shirtless self-portraits of Weiner on Monday — sought to hijack the Sheraton news conference by addressing reporters and fielding their questions. Many observers wondered whether the entire event was a Breitbart gag.

But when Weiner, dour-faced and contrite, took the stage about 4:25, it was clear the situation was serious. He delivered a statement full of “regret” and fielded questions. He apologized to “everyone in the media,” including, after some prodding, Andrew Breitbart by name.

Weiner’s conservative opponents were not satisfied.

“It’s time for Democratic leadership to explain why Congressman Weiner’s actions never aroused any suspicion and why they rushed to his defense while so many Americans were shocked and confused by his bizarre and disturbing behavior,” said Paul Lindsay, spokesman for the National Republican Congressional Committee.

The criticism echoes Pelosi’s critique of GOP leaders in fall 2006, when the scandal of the day was then-Rep. Mark Foley’s instant-message flirtations with congressional pages. Pelosi criticized Republicans for ignoring warning signs.

Foley (R-Fla.) resigned rather than face an ethics committee investigation. More recently, Rep. Chris Lee (R-N.Y.) resigned after sending shirtless photos of himself to a woman he had met on Craigslist.

Weiner on Monday said he had no reason to believe that any of the women he communicated with were underage but allowed that he had only their social media profiles to go on.

Ethics lawyer Stanley Brand said there are no clear-cut congressional rules on how members should behave on the Internet. “We’re in the Twitter era,” he said.

Weiner said Monday that his wife, Abedin, knew of some of his prior online relationships but that he had confessed sending the photo to the college-age woman in Seattle only on Monday morning. His voice quavered as he spoke of his love for his wife, who did not attend the news conference, and as he insisted that she loved him, too.

He expressed confidence that the couple would weather the episode and that voters would appreciate his legislative work and return him to Congress. As for the behavior that has turned a rising political star into a national punch line, he paused at the lectern.

“If you are looking for any deep explanation,” he said, “I don’t have one.”

Staff writers Paul Kane and Felicia Sonmez contributed to this report.