An explosion damaged buildings in Athens' central square early Monday, slightly injuring two people, police said. The blast at Syntagma Square damaged a central post office building - before opening hours - as well several nearby businesses, and parked cars. The square is one of Athens' busiest areas and the center of the city's Christmas and New Year celebrations. Police said the , and occurred following two warning telephone calls made to an Athens newspaper. Authorities had earlier said they believed the blast had been caused by igniting gas canisters. Several vehicles were damaged by glass shards thrown onto the street, windows in nearby buildings were shattered, and two people were hospitalized with cuts. The caller to the newspaper said the nearby National Economy Ministry, which was not damaged, was the intended target of the attack. He did not identify any group behind the explosion. "The people who commit these acts are harming their country," Labor Minister Panos Panayiotopoulos told private Alpha television. "Greece is a peaceful country, this (attack) is hurting that image." Government buildings are frequently targeted in smaller arson attacks, carried out by anarchist groups in Athens - often spurred by political events. The blast coincided with a series of strikes, starting Monday, by Greece's largest unions against plans by Greece's conservative government to change labor rules across the public sector. In June, Greece's Labor ministry was targeted in an overnight bomb attack, which caused extensive damage to the building but no injuries. A shadowy group called itself Revolutionary Struggle claimed responsibility for the blast, and said the attack was in opposition to the government's labor reforms.