NEW YORK (Reuters) - Dozens of pro-Palestinian student protesters arrested in April after occupying and barricading a building at Columbia University in New York City had all criminal charges against them dropped on Thursday, Manhattan prosecutors said at a court hearing.

The hearing at the Manhattan Criminal Courthouse came seven weeks after Columbia administrators called in hundreds of armed and heavily armored police officers to the university's campus in a high-profile law-enforcement response that was broadcast live on national news channels.

Police arrested 46 protesters who had barricaded themselves inside Hamilton Hall, and cleared a weeks-old tent encampment on a nearby Columbia lawn that has inspired similar pro-Palestinian protests at universities around the world.

All 46 protesters, who were arrested on the night of April 30 about 20 hours after taking over the academic building, were initially charged with trespass in the third degree, a misdemeanor.

Stephen Millan, a prosecutor in the Manhattan district attorney's office, told the court on Thursday his office would not prosecute 30 protesters who were Columbia students at the time of the arrest, nor two who were Columbia employees, citing prosecutorial discretion and lack of evidence. A case against another student was dismissed earlier in the month.

Millan said protesters had covered surveillance cameras, and there was insufficient evidence to show that any individual defendant damaged property or injured anyone. No police officers were injured during the arrests, the prosecutor noted. None of the arrested students had any prior criminal history, and all were facing disciplinary proceedings, including suspensions and expulsions, by Columbia.

"All these matters are dismissed and sealed in the interest of justice," Judge Kevin McGrath announced in the courtroom where dozens of defendants and their supporters appeared with keffiyeh scarves around their shoulders. The scarves have become a symbol of the pro-Palestinian movement.

Prosecutors declined to outright drop trespass charges against 13 other people arrested inside Hamilton Hall that night. Two of the 13 were also Columbia students, while the other 11 had no current affiliation with the school, although most were alumni.

The district attorney's office proposed the 13 accept an adjournment in contemplation of dismissal, a provision in New York law that if accepted means the case against a defendant will be dropped and sealed in six months if they are not arrested for another offense in the interim.

All 13 rejected the offer through their lawyers, and due to return to court on July 25, by which date prosecutors must decide if they are willing to proceed to a trial over the trespass charges. Another arrested protester accepted the offer earlier in June.

(Reporting by Jonathan Allen in New York; editing by Donna Bryson and Diane Craft)

By Jonathan Allen