Police arrested about 20 demonstrators on the streets around Woodruff Park late Saturday as the Occupy Atlanta protest took an unexpected turn onto Peachtree Street.

Before midnight, waves of police were moving south down Peachtree, forcing the marchers back. The first rank of police was simply in uniform and carried plastic handcuffs. They were backed by officers in riot gear — face plates, body armor and long nightsticks — and mounted police.

A knot of protesters chanted “Shame! Shame! Shame!” as the police advanced.

The march was a surprise move by the protesters. Nearly 150 went to Woodruff Park, with many saying they intended to be arrested by refusing to leave the park when it closed at 11 p.m. But shortly after 11, the group began emptying out of the park.

A couple of representative members were hauled away with hands strapped, but their move turned into a brief melee when a motorcycle policeman nearly ran over a protester who was marching with others around the park.

The confrontation quickly turned into a shouting match with a line of officers facing off against a chanting crowd of about 150.

A police spokesman said the man who was nearly run over will be charged with attacking the police officer.

Ladie Mansfield, one of the protestors, said group decided to save the city money as a way of making a statement.

“There is a bigger message,” Mansfield said. That is that there are better ways to spend money than arresting peaceful protesters and the city should be sending police to neighborhoods where they were needed.

The protesters quickly folded or dragged more than 25 tents, tables and sleeping bags out of the park as police waited by the dozens at the north end of the park to walk in and start arresting people.

As the crowd left the park they marched around its perimeter, chanting “Take back Wall Street.”

The police made a final show of force by sending a squadron of motorcycles around the park with blue lights blazing and sirens wailing, followed by a squad of about 10 officers on horseback.

The mixed group of protesters included students and working people, mothers and the homeless. Their grievances included foreclosure and bank bailouts, greed, wars and lack of political representation.

They wandered among the 25 or so tents set up in the park, talking, planning for the night’s arrests and talking about the issues that brought them together: joblessness, student debt, foreclosures, two wars, corporate greed and a lack of political representation.

The Rev. Jesse Jackson and other members of the Rainbow/PUSH Coalition went to the park earlier in the evening to talk with the protesters.

About 60 people gathered around Jackson as he told them their movement was an extension of the last movement organized by the Rev. Martin Luther King Jr., the Poor People’s Movement.

“It’s not the size of the crowd, it’s the substance of the discussion,” that is important, he told them.

In an interview, Jackson said the protesters were voicing dissatisfaction with banks, with government policies that favor the rich, with Washington gridlock and lack of action to help average Americans.

“This is the cup running over,” he said. “People can’t take it anymore.”

Temperatures downtown were in the low 50s by nightfall and were expected to dip below 40 toward dawn. Protesters came ready for the cold, bundled up in coats and hats against the wind sweeping across the park.

As they waited for the confrontation to come, one or more protesters Saturday evening stole across the street to the SunTrust building that fronts on the park, lowered the American flag and then ran it back up the flagpole upside down — a widely recognized distress signal.

A short time later, SunTrust security guards came out, took the flag down again, righted it and sent it back up the pole.

The Occupy Atlanta protest began Oct. 7 with a few people and grew into a tent-city occupation of Woodruff Park that continued for nearly three weeks.

The mayor’s office said Thursday that the first occupation of the park cost city taxpayers more than $451,000, most of it in police overtime.

Dispatch editor Angel K. Brooks and staff writer Shelia M. Poole contributed to this article.