(CNN) -- Protesters in Portland rallied in the streets overnight after the city closed a park half an hour ahead of schedule.
"This is a peaceful protest! We have the right to assemble!" demonstrators chanted late Saturday night.
The city closed South Park Blocks park about 8:30 p.m. (11:30 p.m. ET) after "park employees and park rangers were confronted by protesters and unable to complete their jobs," according to Portland Parks and Recreation.
Department spokesman Mark Ross said crews were trying to enforce park rules by clearing structures such as tents. He estimated a few hundred protesters were at the park.
"Immediately (after) they went to the park, they were surrounded," Ross said.
Portland Police Bureau spokesman Sgt. Pete Simpson described the situation as a "hostile environment."
But Jordan Ledoux, media liaison for Occupy Portland, had a different account.
"I was pushed down by a police officer while walking away from them," Ledoux said. "They were not kind."
After the emergency closure, protesters filtered out of South Park Blocks and moved to City Hall and into the streets. Officials warned on a loudspeaker that nearby parks were closed and that trespassers would be subject to arrest, but some returned to South Park Blocks.
More than a dozen arrests were made, primarily for criminal trespass or interfering with police officers, Portland Police Bureau spokesman Sgt. Pete Simpson said.
By 1 a.m. Sunday, "We've basically pulled all our officers out of the area," Simpson said. "Things are pretty calm."
Ledoux and other protesters returned to South Park Blocks by early Sunday morning.
"The First Amendment has been around longer than park ordinances," he said.
Last month, Portland police made more than 50 arrests as they cleared two parks -- Chapman and Lownsdale Square -- of protesters.
Mayor Sam Adams said at the time that "a series of increased drug overdoses... an arsonist that used the camp as camouflage and almost a 20% increase in crime surrounding the encampment" prompted the move.
"All of us are working really hard at keeping the peace and protecting freedom of expression," Adams said in November. "I support a lot of what the encampment stands for ... (But) it shouldn't be focused on port-a-potties and tents and encampments attracting criminal elements. I think this movement needs to evolve."