PHILADELPHIA -- Police briefly detained more than 50 people after they tried to storm the barricades outside the Democratic National Convention on Monday in a show of anger over Bernie Sanders' treatment by party leaders, even as he urged his supporters to fall in line behind Hillary Clinton.
Several hundred Sanders supporters and other demonstrators converged in the sweltering heat on Broad Street and made their way 4 miles to the convention site as the gathering was being gaveled to order, chanting "Nominate Sanders or lose in November!" and "Hey, hey, ho, ho, the DNC has got to go!" They carried signs reading, "Never Hillary," ''Just Go to Jail Hillary" and "You Lost Me at Hillary."
As tensions mounted outside the Wells Fargo Center, police moved metal fences into place and closed the nearest subway station to arriving trains. Fifty-five people were issued citations for disorderly conduct when protesters tried to climb over police barricades at the edge of the security zone surrounding the convention, police said.
The anger reflected the widening rift inside the Democratic Party and the convention hall between Sanders' supporters and Clinton's. Debbie Wasserman Schultz resigned as Democratic Party chairwoman on Sunday over leaked emails suggesting the supposedly neutral Democratic National Committee played favorites during the primaries by siding with Clinton and bad-mouthing Sanders.
Addressing the convention Monday night, Sanders urged his supporters to vote for Clinton, generating a chorus of boos and chants of, "Bernie! Bernie!"
"Based on her ideas and her leadership, Hillary Clinton must become the next president of the United States," Sanders said. "The choice is not even close."
The protests took shape amid a punishing heatwave, with oppressive humidity and temperatures in the mid-90s. Heavy rain and thunderstorms rolled through the area Monday evening, scattering most of the protesters. Forty-one people were treated for heat-related issues or other injuries, including from falls, during protests, fire officials said.
Protester Destine Madu, of Maplewood, New Jersey, said it doesn't matter if Sanders, a Vermont U.S. senator, is calling on his backers to support Clinton.
"He's like a Moses," she said. "He led us to the promised land."
Philadelphia Police Commissioner Richard Ross and other officers walked alongside Sanders protesters on Sunday. Ross said he liked what he saw the day before: a peaceful protest.
Ross -- speaking at the Emergency Operation Center on Spring Garden on Sunday -- said they are ready to handle the masses and don't expect any problems, CBS Philly reported.
"We have done a significant amount of planning with the protesters, as to everyone's expectations, there's and ours," he said.
The citations on Monday capped a day of demonstrations around the city on the opening day of the convention, at which Clinton is expected to receive the party's nomination for president.
About 100 Sanders supporters made their way into Philadelphia by marching across the Ben Franklin Bridge from Camden, New Jersey. Among them was Jim Glidden, a salesman from Batavia, New York. He carried a big sign saying the DNC stands for "Dishonest Nefarious Corrupt."
"Only one guy is telling the truth out there," he said, referring to Sanders. "And the DNC shut him up with lies and cheating."
Another participant in the bridge march, Deborah Armstrong, of Spokane, Washington, said she and her husband went bankrupt because of his health problems, which required a heart transplant.
"I'm Bernie or bust," she said. "I'm not going to have Trump held up to our head like a gun."
The demonstrators espoused a variety of causes, including economic justice, socialism and marijuana legalization. With Sanders out of the race, some of them were backing Green Party candidate Jill Stein.
Tensions rose when about 50 marchers sat down on Broad Street and refused to move unless the Mississippi state flag, which features the Confederate emblem, was taken down from a lamppost. The flags of all 50 states fly from light poles on the street.
Two officers stood in front of the lamppost, not allowing anyone to climb it, as hecklers jeered, "Think for yourself. Be a real man."
City officials later removed the flag.
"The Confederate flag raises strong feelings in our city and across the country," said the city's first deputy managing director, Brian Abernathy.
The four-day convention is far removed from City Hall and the skyscrapers of Center City.
In contrast, the Republican convention, which took place last week in Cleveland, was held in a bustling part of the city. A heavy police presence and fewer than expected protesters helped authorities maintain order. Only about two dozen arrests were made.