In a letter to New York City Police Commissioner Dermot Shea, the president of the New York City chapter of the National Lawyers Guild on Sunday demanded discipline for the NYPD officers who searched and detained NLG legal observers at a protest Thursday in the Bronx.

During the protest against police brutality in Mott Haven, nine legal observers wearing neon green hats marked "National Lawyers Guild Legal Observer" were restrained with zip-tie cuffs and prevented from doing their constitutionally protected work, NLG-NYC President Andy Izenson wrote in the letter.

Moira Meltzer-Cohen, a New York attorney with extensive legal observation experience who also signed the letter, said that while legal observers occasionally get arrested or shoved against cars by police, Thursday's events stood out.

"I don't believe that I have ever observed that level of physicality with that many legal observers all at once," she said.

Several legal observers were thrown to the ground before they were cuffed, including Rex Santus, a rising 2L at City University of New York School of Law.

Santus said he heard one comment from an NYPD officer that indicated the legal observers were being targeted, although officers also insisted they had never heard of legal observers.

"We had asked the NYPD what was going on, why we were detained, and one of them said something to the effect of 'We were told to round up the green hats,'" Santus said.

Before the mass detention of the legal observers, Santus said he was cornered by a group of about eight NYPD officers who seized his notes, which are considered privileged material.

Even though Santus was wearing his neon hat and immediately explained that he was a legal observer, he said the officers claimed to be unfamiliar with the concept.

The officers mocked him and read his notes aloud to him, Santus said. One officer accused him of "illegal countersurveillance of the police," he recalled.

The NLG letter emphasized that legal observers' rights are explained in the NYPD's Patrol Guide. The neon green hat has long been understood to identify a legal observer, Meltzer-Cohen said, and the program has been operating since the late 1960s.

"NYPD officers should be, and have long been, on notice of what the legal observation program is, of who the National Lawyers Guild is, of what legal observers are there to do at protests, of the fact that what they do is constitutionally protected activity, of the fact that their notes are considered privileged, of how to identify them by their bright green hats," she said.

Legal observers were also specifically exempted from the city's 8 p.m. curfew and carried paperwork to that effect, Meltzer-Cohen said, noting that the curfew was apparently serving as the justification for the mass arrests in Mott Haven.

In addition to discipline for the officers and supervisors involved in the actions against legal observers Thursday, the letter demanded that Shea remind all NYPD officers about legal observers' status and role.

Press officers for the NYPD and the New York City Law Department did not immediately respond to requests for comment Monday.

READ MORE: