ATHENS, Greece (AP) — Greece’s antiterrorism squad is investigating after the national riot police headquarters was targeted Monday with a bomb that was defused before it could explode, as police union members protested increasing violence against officers in the force.

Authorities had been alerted after an anonymous caller warned a newspaper that the bomb would explode in 45 minutes and described where it had been planted. That gave bomb disposal experts time to find and defuse the device.

The bomb had been left in a copse near the riot police headquarters outside the city center and close to a university campus, police said. There was no claim of responsibility.

Such bomb attacks have long been associated with domestic far-left militants.

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Many in Greece accuse riot police of brutality against demonstrators during the myriad protests held by leftwing groups or students who often attack police, sparking widespread clashes. Violence is also common during sports events, where fans attack police when not fighting each other.

On Dec. 7, a riot policeman was critically injured by a flare when fans attacked police outside an Athens arena where a volleyball match was being played. An 18-year-old man has been arrested on suspicion of firing the flare, which hit the policeman in the leg, severing an artery.

The attack was linked with soccer hooligans and triggered a two-month ban on fans attending Greek top-flight soccer matches.

Later Monday, about 1,500 uniformed members of the police, fire service and coast guard marched through central Athens to protest the attack on the riot policeman and the bomb incident.

Union members want their jobs to be officially designated as dangerous, which would accord them certain pay and pension benefits.

Earlier Monday, a police motorcyclist died in a crash outside Athens as police were chasing a vehicle following a suspected car theft.