Police fired pepper spray at a surging crowd and handcuffed several demonstrators as skirmishes broke out during a flag-burning in Cleveland outside the Republican National Convention.

It was the most turbulent protest since the four-day convention began on Monday. The chaos on Wednesday prevented delegates and members of the media from getting into the Quicken Loans Arena for the evening's proceedings.

Carl Dix, a representative of the Revolutionary Communist Party, said the group organised the burning of the American flag as a "political statement about the crimes of the American empire. There's nothing great about America".

Police said two officers were assaulted and suffered minor injuries. One officer was seen bleeding from his elbow.

Cleveland police arrested 17 protesters. Two were charged with felonious assault on an officer after they tried to confiscate the flag. The other 15 people were charged with inciting violence.

Burning the American flag is protected by the First Amendment of the US Constitution, which protects the right to free speech.

One man who was in handcuffs stood in the street with his shirt pulled above his shoulders. A woman in a torn shirt also was led away in handcuffs.

Officers, some wearing riot helmets, yelled at the crowd to move back as members of the flag-burning group locked arms amid chants of "It's time, it's time for a revolution." Pushing and shoving broke out, and police began pinning people to the ground.

Cleveland Police Chief Calvin Williams was among a dozen officers pushing people away. He later pulled aside a man with a bandana over his face and told him he needed to check his backpack.

Firefighters extinguished the flag and took it away.

Earlier in the day, blocks away from the arena, a right-wing religious group lifted a banner reading "Jesus is angry with you sinners," while kissing lesbians mocked their message, helping turn Cleveland's Public Square into part-carnival, part-debate floor.

The expansive square was a free-flowing mix of ideas and beliefs along with colourful characters pounding on bongos and wailing on a sousaphone.

The day's demonstrations started with a few dozen people holding banners printed with a red-brick design and forming a human wall to mock Donald Trump's plan to seal off the Mexican border.

"We want to wall off the hate of Trump," said Tim Chavez of Columbus.

A half-dozen Trump supporters defended Trump from attacks by immigration activists.

Police officers used bicycles and their bodies to separate those with opposing views.

Jesse Gonzalez, of Lakewood, a Cleveland suburb, carried a rifle at Public Square while wearing a camouflage-style "Make America Great Again" hat.

"I'm out here to illustrate that not all gun owners, if any or very few, are irresponsible or uneducated," he said.

The city's police chief said he spent three hours on Tuesday evening riding with bicycle officers on patrol and that he waded into one confrontation because he's "still a police officer".

Williams said he plans to show up wherever there are "issues" during the convention.

As of Tuesday evening, about a dozen people had been arrested since the start of the convention.

That includes one person accused of trying to steal a state trooper's gas mask, and three people charged with climbing flagpoles at the Rock and Roll Hall of Fame and hanging an anti-Trump banner.