The police captain, bullhorn in hand, paid no mind to the heckler gesturing and yelling in front of him. His stern command was clear: the young man, and with him the crowd of Occupy Wall Street demonstrators assembled at the plaza on the southern tip of Manhattan, would all have to move. And fast.

They were, the captain told them, breaking the law by standing in a New York City park after closing time. They would be given a little time to vacate the premises, but after that, anyone who does not disperse will be subject to arrest under park rules.

That was the scene last Tuesday at the public space known as the New York Vietnam Veterans Memorial Plaza, where about a thousand demonstrators had descended following a day of street protest called by the loosely organized movement against social and economic injustice known as Occupy Wall Street. It was the end of May Day, and the protesters -- who had flooded the park and conducted an improptu forum earlier -- were now surrounded by hundreds of NYPD officers, who had followed the Occupy march from Union Square.

The captain's threat wasn't hollow. Within minutes, 12 people who had refused police orders to evacuate had been arrested and were being marched, in plastic handcuffs, to a blue-and-white NYPD paddy wagon. They were charged with remaining in a New York City Park after closing without permission, a crime for which late-night joggers, amorous couples and mischievous teenagers are more commonly cited.

The officer's authority to issue that threat, however, is less certain. As it turns out, protesters were not standing in a New York City park at all when they were told to disperse.

The detentions could be chalked up to honest mistake: The plaza where protesters were standing is an open paved space between two skyscrapers in the city's Financial District that is in part a public park and partly a private corporate plaza. And while protesters were arrested in the privately owned section, the border between the two is a simple low wall.

But according to demonstrators, the arrests were made solely to disperse the crowd. No matter what, the police did not act in the right way. They only did that because, in the morning, when those criminal financial institutions opened up, they didn't want us there, said Tarak Kauff, one of the dozen demonstrators arrested.

The police deny that.

All arrests were lawful, and a representative of private property there was sent by property management to act as complainant, said Deputy Commissioner Paul J. Browne, the NYPD's top spokesman.

The presence of the about 1,000 demonstrators at the plaza was the fortuitous result of some confusion at the end of the day's headline march, which took crowds from Union Square to Wall Street down Broadway. The marchers had been prevented by police from reaching Wall Street, and as they navigated the winding, cobblestoned -- and now heavily blockaded -- streets of New York's historic Financial District, they found themselves funneled to the space.

Many of those who walked into the memorial plaza, from around 8:30 p.m. onwards, expressed awe as they took in the sight around them: an open, well-lit, fully paved public space whose main architectural feature is a circular amphitheater-style seating circle arranged around a reflecting fountain and backed by a lit glass wall. In other words, just about the perfect place to host a large public gathering.

The scene quickly turned into a re-creation of the atmosphere from Zuccotti Park, the epicenter of the movement last fall. A general assembly, the term the Occupy movement uses for its anarchic public forums, was called. At the same time, a percussion dance party broke out on the northern side of the plaza, drawing people away from the assembly.

Unbeknownst to protesters, that meant they were actually occupying multiple addresses at the same time. Legally, the part south of the memorial wall where the protesters were holding their gathering is a New York City park officially christened Vietnam Veterans Plaza. North of the wall, the paved area is property of 55 Water Street, a large corporate office building that houses the global headquarters of credit rating agency Standard and Poor's, among others.

Our building line is opposite to the wall, which is at the very edge of city property, said Harry Bridgwood, executive vice president of the New Water Street Corporation, which manages the skyscraper at 55 Water on behalf of its owner, the $30 billion Alabama public pension fund. That distinction is also noted in the official map of the park issued by the city of New York, which is available online.

But shortly after 10 p.m., when demonstrators were gathering around a set of steps squarely on the north side of the plaza, hundreds of police officers ignored the technical boundary and descended on them in lockstep. Many demonstrators ran.

Tarak Kauff was among the gathered protesters. He is part of the recently formed Veterans Peace Team, a group of former armed forces personnel who, Kauff said, have volunteered to be somewhat of a bodyguard to the Occupy movement.

Asked why he stayed after the crowd scattered, Kauff said that his group had every right to be there. And the crowd had every right to be there.

Eric Arnone, the Manhattan lawyer, agreed: If they were in the part that's privately owned, the police don't have the authority to summon them under the parks regulation, he said, adding that it is doubtful whether the police even had the authority to order people to leave at all.

According to Arnone, the use of the parks regulation to arrest demonstrators is rare. Protesters are most commonly cited for disorderly conduct, a violation of the penal code; breaking the parks regulation is a more serious offense, officially a misdemeanor.

The police wouldn't be able to vacate until they have reason to believe people were there trespassing. And then they'd arrest them for trespassing, he said.

That could have happened. Speaking from a polished cherrywood table at his office's conference room two days after the arrests, Bridgwood, the site manager, emphatically said his company would have been a willing complainant against the protesters. But the police never asked him officially, even though according to the statement from the NYPD, there was a complaint from the company.

New York City Councilman Ydanis Rodriguez, who witnessed the arrests Tuesday night and was arrested himself while taking part in the Occupy demonstrations last November, said that while he cannot speak of the specific issues of the Veterans' Plaza situation, there have been a number of arrests where there wasn't any ground for people to be arrested. For example, earlier last month, the police cleared out a sleepful protest, staged by activists sleeping on the sidewalk across from the New York Stock Exchange, even though lawyers friendly to the movement had noted demonstrators were protected by a 2000 federal court decision that defined such action as protected free speech. Rodriguez is suing the city over his arrest last fall, which he says was unlawful.

As for the 12 arrested Tuesday at the memorial plaza, their day in court is set for July 11.