Greek police on Saturday said they had arrested four men accused of possessing explosives, a week after a bomb exploded outside the labour ministry in Athens.

The bomb exploded early on February 3 but caused no casualties. It had been placed in a backpack and hung on railings opposite the ministry.

The suspects were arrested on Friday “for construction and possession of explosives and incendiary material,” police said in a statement.

The arrests were not immediately tied to the blast, but one of the men had been linked to an extremist group, the statement added.

Greek media identified the group as the Conspiracy of Fire Nuclei, an anarchist outfit whose members served long prison terms for letter bomb attacks.

A man who anonymously called a Greek daily to warn about the impending February 3 blast claimed to be a member of Revolutionary Class Self-Defence, a previously unknown organisation.

Attacks on public establishments, banks and diplomatic representations have been recurrent in Greece for years. They are generally attributed to extreme left-wing or anarchist movements.