ATHENS—Two security guards were slightly injured Sunday in a bomb blast at an indoor suburban shopping mall north of the Greek capital, police said.

The blast caused only modest property damage, police said, and was preceded by warning calls to two Greek media organizations almost an hour beforehand. This allowed authorities to vacate the building, where most shops were closed, and seal off the area. Urban and suburban rail services on the lines astride the building were suspended.

The blast comes after recent attacks against journalists and the country's governing center-right party that has awakened fears of a new terror campaign by radical Greek leftist groups. The police's antiterrorism squad has been called in to investigate the incident, a police official said.

Police secure an area on Sunday near the shopping mall north of Athens where a bomb exploded. Reuters

Last Monday, unidentified gunmen fired shots at the offices of Greece's ruling conservative party, days after a previously unknown group claimed responsibility for a series of bomb attacks against five prominent journalists. The brother of Greece's government spokesman was also targeted in a separate arson attack a week ago Sunday.

The attacks resemble past strikes by Greece's shadowy left-wing urban guerrilla groups and appear to be linked to recent efforts by Greek authorities to crack down on self-styled anarchists, illegally occupying buildings in the center of Athens. In the past few weeks, the country's coalition government, dominated by the conservative New Democracy party, has raided three of roughly a dozen buildings controlled by the groups in the Greek capital.

In December, police cleared out one such building, known as the Villa Amalia, where anarchist groups had been squatting for 22 years. Police said the building served as a staging ground for the violent demonstrations that have rocked Athens since the start of its debt crisis three years ago.

Although police said that the bomb used in the mall attack was a relatively low-grade explosive, it also represents a rare attack on a commercial—rather than political—target. The timing of the blast and placement of the explosive, on the first level of the building, has also raised unease, possibly signaling a new trend or shift away from past such attacks.

Previous bombings by Greek left-wing groups stretching back over several years have generally aimed at minimizing casualties. Many are conducted in the middle of the night, tend to target police or other symbols of authority and aren't aimed at the ordinary public or tourists.

Write to Alkman Granitsas at