Supporters of Bernie Sanders gathered outside City Hall in Philadelphia on Monday. Most vowed to never vote for Hillary Clinton, despite Sanders's encouragement to do so while speaking a few blocks away. (Alice Li,Jayne Orenstein/The Washington Post)

“Never Hillary” and “RIP, DNC” and “Bernie or Bust” read the placards as thousands of protesters, vociferously not with the unity-first program of the Democratic National Committee, gathered here Monday to express dismay with their party, their presumed presidential nominee and a political system they consider corrupt.

Many left-wing groups skipped last week’s Republican National Convention in Cleveland and focused instead on this city and the Democrats. Many remain supporters of Bernie Sanders, and they were joined by people protesting police brutality, advocating immigrant rights and pushing for gun control.

The Revolutionary Communists chanted that Hillary Clinton should be in prison. Other protesters planned a mock trial of the Democratic candidate. Another group prepared to erect “tombstones for democracy” in a park.

Sanders supporters repeatedly declared that there is no chance they will vote for Clinton, whose nomination many described as illegitimate. Sanders got a sense of that himself when, speaking midday at a convention center, the crowd rejected his call to support his former rival.

“I’m a Never Hillary person. Because she’s corrupt. She represents everything we’re against,” said Luigi Costello, 60, of Sarasota, Fla., as he held a makeshift peace symbol at a late-morning protest at City Hall.

Although police braced for clashes between supporters and detractors of Republican nominee Donald Trump at the Republican convention in Cleveland, the confrontations never materialized. Philadelphia is another story: The crowds are here — and far larger than the ones in Cleveland. Some activists hope to disrupt the convention. Others plan to be arrested. As of mid-afternoon Monday, no arrests had been made, police said.

The one constant was the sun overhead. The heat index reached triple digits. Local authorities sounded as worried about people suffering heat stroke as they were about civil unrest and violence. Heavy storms raced toward the city, poised to strike in the evening as the convention hit prime time.

More protests are expected Tuesday. The unhappiness of the Sanders supporters could prove a challenge for Democratic leaders hoping to pull off a unified convention, something the Republicans were unable to do last week.

During a joint rally at City Hall that stretched for more than two hours, Sanders supporters circulated an “open letter” from his delegates in which they urged super-delegates to abandon Clinton and vote for Sanders.

“You’d have to be crazy not to be worried about the possibility of Trump or Hillary becoming president,” said Amanda Sullivan, 35, a computer programmer from Weston, Fla.

Sullivan, who said she’ll support Green Party nominee Jill Stein, argued that it was unlikely that support for a third-party candidate would result in Clinton losing to Trump. But, she added, Trump is “actually less of a threat to democracy than Hillary.”

Ryan Hoke, 23, a college student and self-described conspiracy theorist, said his online research had proved that the campaign was a “stitch-up.”

A young anti-Hillary Clinton protester is arrested outside the Wells Fargo Center on the first day of the Democratic National Convention in Philadelphia on July 25, 2016. (Ricky Carioti/The Washington Post)

“You look at Trump and realize he’s the ultimate fear-porn false flag,” he said. “This system has been rigged so everyone feels they have to vote for Clinton, and we’re just not going to fall for that.”

Braving the heat in a full Trump costume, Eric Varlo, an Occupy Denver activist, made a similar argument: “Even a Hollywood script writer wouldn’t manage something this good.” Despite strong objections to Trump’s views, Varlo said, Trump might make a better president. “With Hillary, you know you’re voting for lies. At least Trump is an unknown quantity,” he said.

Said Jeremy Dolan, 24, of St. Petersburg, Fla., a Sanders supporter who said he votes in Democratic primaries and supported President Obama in 2012, “We did eight years of Bush and nothing that bad happened, so we can deal with four years of Trump.”

When asked about the Iraq War and the USA Patriot Act — two decisions during George W. Bush’s presidency that many on the left despise — Dolan shook his head.

“What I say to that is that I don’t negotiate with terrorists,” Dolan responded. “I’ll never vote for Clinton.”

People were buzzing about the leaked DNC emails and the under-pressure resignation of Rep. Debbie Wasserman Schultz (Fla.) as party chairman. “One down, one to go!” was a popular chant — although few here realistically think Clinton will go the way of Schultz. Another chant: “Hell no, DNC, we won’t vote for Hillary!”

“It’s not over till the super-delegates have voted,” said Cheryl Miller of Austin.

Rachel Kessler of Bristol, Pa., said, “We believe the primary was rigged against [Sanders]. The WikiLeaks emails prove that.”

Tracy Graunstadt drove from Michigan for the protest, saying she opposes the two-party system. “I want people to see that we’re not going to give up on the revolution. We’re not going to give up on Bernie,” she said. “We’re not blind to the corruption of the DNC.”

Philadelphia police officials said they’ve changed strategies since 2000, when the Republicans held their convention here and many people were arrested. Police will not use tear gas and have decriminalized certain protest-related nuisance crimes, according to the Philadelphia Inquirer, which quoted the mayor saying the city’s goal is to make no arrests.

One flashpoint of protests had been the state flag of Mississippi, which contains a Confederate battle flag symbol. State flags had gone up along the street ahead of the convention. The mayor’s office decided Monday to remove the Mississippi flag after hearing complaints from neighbors.

“Rip it down! Rip it down!” protesters chanted late Monday afternoon as they marched along Broad Street. Suddenly a city truck known as a cherry picker arrived on the scene, escorted by police. A city employee removed two Mississippi flags from opposite sides of the street as protesters cheered — a small victory, the activists felt, for People Power.

Lateshia Beachum and Kayla Epstein contributed to this report.