Kevin Johnson, and Aamer Madhani

USA TODAY

PHILADELPHIA -- The most immediate threat to the thousands of visitors and protesters converging here for the Democratic National Convention is likely to be the extreme heat expected to extend through the four-day gathering.

With temperatures hovering near triple digits, Mayor Jim Kenney said Sunday that the city is taking extraordinary precautions, deploying medics to each protest march along with pallets of water and opening fire hydrant sprinklers to keep demonstrators cool.

"It is important to take the extreme heat seriously," Kenney said.

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The heat could affect a range of security operations, Kenney and Police Commissioner Richard Ross said, including whether the high temperatures and stifling humidity discourage some of the estimated 50,000 protesters expected each day.

In addition to the medics and water supplies, city officials are establishing two medical tents and two water misting tents to tend to protesters and visitors.

"It's going to be tough with the heat," Ross said, adding that officials know of no other credible threats to the convention involving groups or individuals.

No arrests were reported during pre-convention demonstrations downtown Sunday afternoon, and no citations were issued involving convention-protest activity, Ross said.

Kenney said officers would be using civil citations in an attempt to cut down on the the number of arrests during the convention, when appropriate, to avoid unnecessary detentions.

The largest of Sunday's demonstrations featured hundreds of supporters of former Democratic presidential contender Bernie Sanders. The group marched five miles in the blistering heat, from City Hall to FDR Park, a sprawling green space near convention headquarters at Wells Fargo Center that is designated as a demonstration site this week.

Police shadowed the peaceful march all along Broad Street, as city officials distributed water and offered encouragement to the marchers, some of whom guided a giant paper-mache likeness of Sanders the entire distance.

James Bennett, 63, and Chris Horton, 72, of Worchester, Mass., traveled to Philadelphia with the hopes that their voices would help persuade super delegates to get behind Sanders. Short of that, they said, they held on to hope that Sanders would abandon his endorsement of Clinton and run as an independent.

The recent leaks of Democratic National Committee emails from the primary season that appeared to show senior staffers favoring Hillary Clinton over Sanders, and suggesting that the Vermont senator’s religious beliefs should be questioned, underscored that the party had stacked the cards in Clinton’s favor, they said.

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“I wouldn’t want to be in Bernie Sanders shoes,” said Bennett, a longtime Democratic party supporter who said he is uncertain whether he’ll vote for Clinton if she seals the nomination as expected this week. “(The Democratic Party) made him well-known. If he hadn’t run as a Democrat, he still wouldn’t be known. I’m torn.”

But Horton argued Sanders' backers owe Clinton nothing.

“These emails show they conspired against him,” Horton said. “They prove he didn’t get a fair shake.”

The march, which ended about 5 p.m., offered the first major test of the massive security operation, which authorities said has accounted for a range of worst-case scenarios.

In response to the Bastille Day terror attack earlier this month in France, Ross said the security plan was tightened in recent days to blunt a possible assault by a large truck. More than 80 people were killed in the French coastal city of Nice when a 31-year-old Tunisian man plowed his delivery truck into a large crowd that had gathered to view fireworks along the scenic city's main promenade.

"We would be remiss if we didn't take extra precautions," Ross said.

The commissioner declined to detail those contingencies, other than to note additional barricades had been placed along strategic routes.

Final tally: Just 24 convention-related arrests in Cleveland

Philadelphia deployed a team of police officials to Cleveland to gather information about security operations during last week's Republican National Convention, where far fewer protesters than expected participated in demonstrations there.

In Cleveland, police reported just 24 arrests during the four-day Republican gathering. One botched flag burning accounted for 17 of those detentions.

"I don't know what will happen here," Ross said. "It's way too early to get into numbers."

Expect heavy security at DNC in Philadelphia

Kenney said the early protester estimates were gathered from permit applications. The mayor said it is possible, as occurred in Cleveland, that only a fraction of those numbers materialize.

"I am cautiously optimistic that things will go well,'' Ross said.

The commissioner said security officials have learned a great deal from past major events, including last year's visit of Pope Francis, which drew 1 million people to the city.

"The pope's visit was this (event) times five or six,'' Kenney said.

He urged the people of the city to make good on its reputation as the "City of Brotherly Love."

"Let's show some love,'' Kenney said "The country is hurting, it needs some hope, it needs some energy.''