Suspected anti-globalisation protesters have attacked a government building which houses the federal mint and defence ministry offices in the Swiss capital, Bern.
Nobody was injured in the blast, which caused minor damage.
The Federal Prosecutor's Office said investigators found traces of explosives near the federal mint.
The quantity of explosives used is not yet known, said spokesman Hansjürg Mark Wiedmer.
Authorities have launched a formal investigation into the incident, which comes just days ahead of the G-8 summit in the French resort of Evian, on the shores of Lake Geneva.
In a fax sent to Swiss media, an anonymous group claimed responsibility for the attack.
It said the blast was designed to draw attention to the "security strategy" of the Swiss authorities during the upcoming G-8 summit, due to start on June 1.
The Swiss defence minister, Samuel Schmid, condemned the attack, saying such an act was not "constructive" to fostering dialogue between opposing groups.
Some 100,000 anti-globalisation protesters are expected to descend on western Switzerland during the summit.
Security is expected to be tight, especially in the cities of Geneva and Lausanne where demonstrations have been planned.
Meanwhile, on Sunday night the home of a defence ministry employee was splashed with paint, however, it is not yet known whether these two incidents are linked.
swissinfo with agencies
Suspected anti-globalisation protesters attacked the Swissmint building in Bern.
Investigators found traces of explosives near the building which also houses Defence Ministry offices.
The blast caused minor damage and nobody was injured in the attack.
An anonymous fax stated the blast was in protest at security measures for the upcoming G-8 summit in Evian.