Boston Police report arresting a Mattapan man who allegedly boarded a bus armed with a semi-automatic weapon at Ashmont station this afternoon.

Dereck Gray, 28, was arrested on a bus on Talbot Avenue after they received a tip a guy in a blue hoodie and a Red Sox cap had gotten onboard with the weapon around 2:10 p.m., police say. He was charged with unlawful possession of a firearm, unlawful possession of ammunition and carrying a loaded firearm.

According to a police statement: "While on Talbot Avenue, officers observed the described bus heading in their direction and directed the driver to stop the bus. Officers boarded the bus and therein saw an individual in the back of the bus matching the suspect description. Officers approached the suspect and as they did so, the suspect stood up, reached in his waistband and discarded a black object on the floor. Officers quickly detained the suspect and recovered a semi-automatic loaded firearm right where the suspect had been dropping the item he discarded from his waistband."

Gray will be arraigned in Dorchester District Court on Friday, police say.