The Police Department will be on heightened alert in coming days in response to a warning about possible terrorist activity that could occur the day before the presidential election, Mayor Bill de Blasio said on Friday.

He stressed that the information about the possible threat, passed on several days ago by federal authorities, was very vague, but that officials were nonetheless taking precautions.

“It’s not at all clear how credible this is, and it’s not overly specific information so far,” Mr. de Blasio, a Democrat, said during his weekly radio appearance on WNYC. “We are watching carefully. We are certainly in a vigilant position.”

The city was informed of the possible threat in a routine intelligence bulletin from the Federal Bureau of Investigation and the Department of Homeland Security, according to a federal law enforcement official who was not authorized to discuss the matter and spoke on the condition of anonymity. The unclassified bulletin said that the terror group Al Qaeda might be planning attacks for Monday, the day before the election, in three states, New York, Texas and Virginia, the official said. He said that officials were skeptical about the credibility of the information.