Republicans arriving in Cleveland next month to nominate Donald Trump will be greeted by as many as 6,000 protesters on the first day, a noisy coalition of dozens of groups, including Black Lives Matter and the Workers World Party. The demonstrators intend to ignore restrictions keeping them far from the delegates, raising fears the violence that accompanied some of Trump’s rallies will be magnified on a mass scale.

Two marches along routes the city has not authorized are planned for the convention’s opening day, July 18. Organizers say they want to avoid violence. But they are also gearing up for confrontation with the police, including training in civil disobedience.

“If there are people willing to put themselves on the line to be arrested, so be it,” said Deb Kline, a leader of Cleveland Jobs With Justice, one of the groups that will march.

A week later, as Democrats pour into Philadelphia, so will an army of Bernie Sanders supporters planning Occupy Wall Street-style protests against what they call the “fraudulent” nomination of Hillary Clinton. One group, Occupy DNC Convention, is circulating information about protecting oneself from tear gas by wearing a vinegar-soaked bandanna and swim goggles.

The divisive nominating contests have produced countercurrents of deep resentment in both parties. And next month, that bitterness could spill into the streets in large protests just when Republicans and Democrats — and the host cities — are trying to present images of unity to the country.

Mass demonstrations have occurred at nearly every modern political convention, perhaps none more disastrously than at the 1968 Democratic convention in Chicago, where street riots contributed to Hubert H. Humphrey’s loss that November.

There is no chance of that level of violence, officials in both cities insist, because modern policing has evolved to be less confrontational. Philadelphia is considering issuing $100 summonses to marchers blocking highways or failing to disperse, rather than arresting them.

Protest organizers in Cleveland say that by imposing strict rules to contain demonstrators, the city is only increasing the likelihood that unsanctioned protests will erupt, leading to violent clashes with the police or between Trump supporters and opponents.

Protest groups say the restrictions are less for safety than to minimize their visibility, and the American Civil Liberties Union of Ohio has already sued the city over the rules, saying they violated free speech rights. The rules limit marches to 50 minutes, mostly during the mornings, hours before the delegates convene, and to a route distant from Quicken Loans Arena, the convention site, passing over what protest organizers call “a bridge to nowhere.”

“The parade route is very short and it goes over a bridge, which means no one can see them,” said Christine Link, executive director of the American Civil Liberties Union of Ohio.

Some protest leaders said they would ignore the restrictions and march as close as possible to the arena, which will be wrapped in a security perimeter set by the Secret Service.

“They made this plan to frustrate and aggravate people who want to demonstrate within their rights against the Republican agenda,” said Thomas Burke, a leader of the Coalition to Stop Trump and March on the RNC.

Expecting thousands for a protest on the convention’s opening day, Burke said he would ignore the official parade route and march from Cleveland’s Public Square to within “sight and sound” of the arena.

“If the police respect our march, I think we have a family-friendly event,” he said. “If people try to provoke and attack our march, I’m pretty sure people will take care of themselves. We’re very well organized.”

Even the organizer of a pro-Trump rally said the city’s rules increased the chance of violence by confining all demonstrators to the same route.

“You stick everybody together face to face, you’re going to have a problem,” said Tim Selaty, organizer of a group called Citizens for Trump, which expects 5,000 to attend. “They have created a much more dangerous situation.”

After the 2004 Republican convention in New York, the city paid out millions of dollars to settle claims of civil rights violations after the police detained 1,800 people, including dozens swept up by long orange nets.

A spokesman for Mayor Frank G. Jackson of Cleveland defended the prescribed march route and said the police have trained for 18 months to deal with protesters.

“Our anticipation is everyone is going to come and do what they’re supposed to do and will be able to have a wonderful time,” said the spokesman, Dan Williams. He added that protesters who chose other march routes would not be arrested as long as they used sidewalks and did not block traffic.

The potential impact on Trump and Clinton as they formally enter the general election is asymmetrical.

In Cleveland, Trump — who will be confronted by left-wing demonstrators, not fellow Republicans — could potentially benefit from scenes of mayhem that allow him to call for law and order and project strength, as he did recently when opponents punched his supporters and burned their “Make America Great Again” hats in California. Street chaos, if it occurs, could overshadow disunion in the convention hall as an increasing array of party leaders nervously break ranks with Trump.

Democratic leaders are worried about emerging from their convention with an unmollified “Bernie or Bust” contingent whose protests could provide jarring split-screen images as the party seeks to rally around Clinton.

Philadelphia is giving the protesters more leeway than Cleveland is, allowing daylong rallies of Sanders supporters in Franklin Delano Roosevelt Park facing the convention site, the Wells Fargo Center, on all four days of the convention, July 25-28.

Bill Taylor, an organizer of the rallies, said he expected up to 250,000 people still angry that Sanders lost the Democratic nomination by what they believe is a rigged system. Since the end of the primaries, as leading Democrats like Sen. Elizabeth Warren and labor groups like the AFL-CIO have endorsed Clinton, Sanders has held off. Last week, he told supporters he would take his movement and “more than 1,900 delegates” to the convention.

Taylor, a Philadelphia resident with no official ties to Sanders, received a request from the campaign to use one of his permits so Sanders could hold a rally in Roosevelt Park on Sunday, July 24, the convention’s eve.

Even as Clinton effectively clinched the nomination by securing a majority of delegates, many Sanders supporters insist there were fraud and voter suppression.

“We feel like democracy has been stolen,” said Laurie Cestnick, founder of a Facebook group called Occupy DNC Convention, which has more than 25,000 members.

Her group is a hub for ride shares to Philadelphia, information about “Bernie Buses” and tent camping and a friendly warning not to use BernieBNB, a house-sharing app that is already overwhelmed.

Cestnick’s application for a permit for up to 15,000 people to march on July 25 is pending with the city.

Taylor’s group, March for Bernie at DNC, plans to demand the resignation of the Democratic Party chairwoman, Rep. Debbie Wasserman Schultz of Florida; the end of the use of superdelegates, or party leaders who get nominating votes and have overwhelmingly backed Clinton; and universal open primaries. Independents strongly favored Sanders over Clinton, but could not vote in the many closed primaries, where only registered Democrats could participate.

If the demands are not met, Taylor warned, die-hard Sanders supporters plan to leave the Democratic Party. Polls show as many as 30 percent of Sanders’ backers say they will not vote for Clinton.

To symbolize their view of the party, Taylor said, “we plan on marching coffins down Broad Street every day.”

© 2016 The New York Times Company