A letter bomb exploded at the office of the Swissnuclear nuclear agency in the northern city of Olten on Thursday morning, injuring two people, police said.

The explosion occurred as staff were arriving to work, police spokesperson Thalia Schweizer stated.

According to her, two employees were hospitalized with light injuries, AP reported.

Swissnuclear represents Swiss national national power companies Axpo, Alpiq and BKW.

Opponents of nuclear power in Switzerland have become more vocal in recent weeks, after the devastation of the Fukushima nuclear power plant in Japan caused by the March 11 earthquake.

Switzerland has five nuclear reactors.