OAKLAND, Calif. – The display of police force in Oakland, Calif., and Atlanta has unnerved some Occupy protesters.

While demonstrators in other cities have built a working relationship with police and city leaders, they wondered on Wednesday how long the good spirit would last and whether they could be next.

Will they have to face riot gear-clad officers and tear gas that their counterparts in Oaklandfaced Tuesday? Or will they be handcuffed and hauled away in the middle of the night like protesters in Atlanta?

“Yes, we’re afraid. Is this the night they’re going to sneak in?” said William Buster of Occupy Wall Street, where the movement began last month against what they call corporate greed.

An Iraq war veteran marching with demonstrators suffered a cracked skull in the chaos between officers and protesters in Oakland, further raising concern among some in the movement. Scott Olsen, a 24-year-old Marine veteran, was in critical condition Wednesday after he had been struck, said a spokesman for Highland Hospital.

The message, meanwhile, from officials in cities where encampments have sprung up was simple: We’ll keep working with you. Just respect your neighbors and keep the camps clean and safe.

Business owners and residents have complained in recent weeks about assaults, drunken fights and sanitation problems. Officials are trying to balance their rights and uphold the law while honoring protesters’ free speech rights.

“I understand the frustration the protesters feel … about inequity in our country as well as Wall Street greed,” Baltimore Mayor Stephanie Rawlings-Blake said. “I support their right to free speech but we also have rules and laws.”

Some cities, such as Providence, R.I., Atlanta and Trenton, N.J., are moving ahead with plans to evict activists. But others, from Tampa, Fla., to Boston, say they will continue to try to work with protesters to address problems in the camps.

In Oakland, officials initially supported the protests. Mayor Jean Quan said that sometimes “democracy is messy.”

But tensions reached a boiling point after a sexual assault, a severe beating and a fire were reported and paramedics were denied access to the camp, city officials said. They also cited concerns about rats, fire hazards and public urination.

Demonstrators disputed the city’s claims, saying that volunteers collect garbage and recycling every six hours, that water is boiled before being used to wash dishes and that rats have long infested the park.

When riot gear-clad police moved in early Tuesday, they were pelted with rocks, bottles and utensils from people in the camp’s kitchen area. They emptied the camp of people, and barricaded the plaza.

Protesters were taken away in plastic handcuffs, most arrested on suspicion of illegal lodging.

Demonstrators returned later in the day to march and retake the plaza. They were met by police officers in riot gear. Several small skirmishes broke out and officers cleared the area by firing tear gas.

The scene repeated itself several times just a few blocks away in front of the plaza.