PHILADELPHIA — Diehard supporters of Sen. Bernard Sanders stormed media tents and staged a sit-in Tuesday night minutes after Hillary Clinton officially claimed the party’s presidential nomination, injecting another round of drama into the convention and pouring cold water on claims Democrats are fully uniting behind their White House nominee.

Hundreds of Sanders backers, most carrying signs backing their candidate, conducted what seemed to be a well-coordinated march from the Wells Fargo Center to the media tents just outside the arena. They said that party officials were trying to confiscate Sanders signs inside Wells Fargo, where high-profile Democrats were speaking about united efforts to beat Republican Donald Trump in November.

The protesters chanted, “This is about a revolution that has to continue, this is about a movement, power to the people,” and other slogans.

“We are the people. United together, we’ll never be defeated,” the crowd chanted as Pennsylvania State Police assembled just outside the media tent to try to maintain order as the Sanders crowd virtually took over the area.

Many of the Sanders backers then sat down throughout the press area and said they refuse to accept Mrs. Clinton as their nominee. Hundreds of reporters descended on the crowd, creating a chaotic scene. Police officers temporarily restricted access in and out of the press area.

“I’m not okay with this. This is not over. This is just the beginning,” said Robert Satiacum, a Washington state Sanders delegate.

By about 8:30 p.m., the protest largely had fizzled out and most of the demonstrators had exited the press tents.

But the incident casts a shadow over Mrs. Clinton’s official claim on the presidential nomination and again showcases that the Democratic Party, despite claims to the contrary, still has not healed from the wounds of a bitter primary fight.

Clashes between the Sanders camp and party leaders have dominated the convention through its first two days.

The Sanders supporters are still stung by revelations that Democratic National Committee leaders actively worked against Mr. Sanders in the party primary, contrary to repeated claims of neutrality by Chairwoman Debbie Wasserman Schultz at the time. They believe they’ve been wronged and that the election has been stolen from their candidate, a Vermont senator who took Mrs. Clinton farther in the primary than virtually anyone anticipated.

Even the ouster of Ms. Schultz hasn’t been enough to quell Sanders delegates, some of whom have vowed to leave the party and flatly say they will not vote for Mrs. Clinton in November.

Several delegates said they had planned to walk out before the nominating process took place unless their voices were heard.

Michael Lopez, a Sanders delegate from North Dakota, said that he and others were ordered to put away their pro-Sanders signs until after the conclusion of the nomination process.

“We’re being told, ‘Put away your signs or you’re going to be arrested,’” Mr. Lopez said.

Raina Whiting of Hawaii said that she was disgusted by what she described as the unfair playing field.

“This is a response to our voices not being heard,” she said. “If Hillary had received the nomination fair and square, that would be one thing. But that’s not what happened.”

Sanders supporters said the protest was organized with the help of Shyla Nelson, a Sanders delegate from Vermont.

“The point is that the whole thing is rigged,” said Earl Hatley, a Sanders delegate from Vinita, Oklahoma. “It’s not that we’re bad losers.”

He said the protest was spontaneous, and culminated after the roll call vote.

“We were hoping that somehow, something would happen (at the roll call),” Mr. Hatley said. “We had a champion … it was as if one of us was running. If the DNC had played fair, we’d probably see Bernie elected. That’s why we’re [expletive].”

Minutes before, Mr. Sanders himself moved to nominate Mrs. Clinton by acclamation, seemingly acknowledging once and for all that his campaign was over and that it was time for the party to unite. Mr. Sanders and other leading progressives also begged for unity during prime-time addresses Monday evening.

Polls have shown that a vast majority of Sanders supporters already have lined up behind Mrs. Clinton, but some Sanders delegates will not fall in line.

“Bernie supporters all over the country have decided to exit the Democratic Party,” said Kimberly Tucker, a Sanders delegate from Chesapeake, Virginia. “What I understand is they are leaving the Democratic Party … I’m here in solidarity.”

⦁ David Sherfinski contributed to this report.

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