Thousands of demonstrators took to Philadelphia's sweltering streets Sunday, chanting and beating drums in the first major protests ahead of the Democratic National Convention, as the city wilts during a heat wave.

Throngs of clean-energy activists jammed a major thoroughfare in their 1.6-kilometre march from Philadelphia City Hall to Independence Mall, near the Liberty Bell. They held anti-fracking and anti-pipeline signs, some with illustrations like a train surrounded by a fireball and the words "No Exploding Trains." Others held "Bernie or Bust" signs.

Sam Miller, 82, travelled from Erie, Pa., to join the march that stretched several blocks and across a wide street as temperatures in the city soared. He said he was inspired because "fracking is invading Mother Earth."

Like in Cleveland, police were using bicycles as barricades along the streets, and volunteers were handing out water to marchers. Shoppers came out of stores to watch the march like a parade.

Chants of "Bernie! Bernie!" were met by counter echoes of "Hillary! Hillary!"

Bernie Sanders supporters expect about 3,000 protesters will join them in a march from City Hall to a park near the convention site in the afternoon.

Some who demonstrated were environmental protesters and held anti-fracking and anti-pipeline signs. (Alex Brandon/Associated Press)

City under 'excessive heat' warnings

The heat wave that descended on the city was showing no mercy, with temperatures expected to reach the upper 30s Celsius and the city under an "excessive heat" warning by the National Weather Service. It's expected to peak Monday, the convention's first day, with temperatures possibly hitting 38 C.

Crowds braving the weather could take advantage of "misting tents" and free water, compliments of the city.

Mayor Jim Kenney warned people to limit time outdoors and said demonstrations would be put on hold in the event of thunderstorms.

The first march Sunday had the perennial problem in Philadelphia: parking. DC to DNC organizer Ed Higgins said finding parking spots and organizing the crowd delayed his group's march, which got going about an hour late.

Protests and demonstrations are planned throughout the city during the convention, and some of the largest start about six kilometres north of the arena where the convention is being held. (John Minchillo/Associated Press)

The group of about a dozen marchers walked on sidewalks from near the convention site up toward the Liberty Bell. One played This Land is Your Land on bagpipes as the group wove around people eating brunch at sidewalk cafes and others walking dogs.

Protests and demonstrations are planned throughout the city during the convention, and some of the largest start about six kilometres north of the arena where the convention is being held.

In Cleveland last week, most protests during the Republican National Convention were concentrated in a tight, 180-hectare zone downtown. A heavy police presence and fewer than expected protesters helped keep the calm. There were only about two dozen arrests and no significant injuries.

Sanders responds to leaked DNC emails

Philadelphia's protests come as Sanders called for the head of the DNC to step down after leaked emails suggested the party played favourites during the presidential primary.

Emails posted to the website Wikileaks show that at least some DNC officials were looking at ways to undercut Sanders' campaign, including questioning his religious beliefs.

Sanders told ABC's This Week Sunday morning that the party chairwoman, Debbie Wasserman Schultz, should resign immediately.

Later on Sunday, Wasserman Schultz said in a statement she will step down at the end of this week's convention. She still plans to fulfil her duties formally opening and closing the convention in Philadelphia. She also said she will speak at the four-day gathering.

More than 5,000 delegates are among the 50,000 people set to attend the gathering at the Wells Fargo Center in South Philadelphia, which is expected to culminate with Clinton being named the party's official nominee for president.

The former secretary of state and first lady has named Senator Tim Kaine of Virginia as her running mate.