The two female employees of Swissnuclear were taken to hospital with superficial burns and hearing damage, a police spokesman said, adding police did not yet know who had sent the parcel.

Police cordoned off the office of Swissnuclear on the fourth floor of a building in the northern town of Olten. The police spokesman said they had forensic specialists on the ground.

Earlier this month, Switzerland suspended the approvals process for three new nuclear power stations so that safety standards could be reviewed after Japan's earthquake and tsunami damaged the Fukushima nuclear power plant.

Swissnuclear says it works to promote the safe and efficient use of nuclear power and represents Swiss utilities Alpiq, Axpo, BKW, CKW and EGL, which run the nuclear plants that produce about 40 percent of Swiss electricity.

Olten is also home to the headquarters of Alpiq, where about 50 Greenpeace protestors held a demonstration calling for the company to withdraw its application to build a new nuclear plant.

A police spokesman said they were examining whether there was any connection between the explosion and the demonstration.

Greenpeace said it had nothing to do with the attack. "We are shocked that such action can be used for political purposes. Greenpeace is committed to non-violent protest," said energy campaigner Florian Kasser.

The center-left Social Democrats and the Greens are calling for Switzerland to abandon nuclear power after the Japan disaster but Energy Minister Doris Leuthard has cautioned against a hasty decision, warning that would mean more gas power stations which would lead to a rise in carbon emissions.

In 1990, Swiss voters backed a 10-year moratorium on the building of nuclear power plants but they rejected extending the freeze in 2003, opening the way for the government to consider new plants to replace those that need retiring.

Last month, voters narrowly approved the building of a new plant in Muehleberg to replace the old one there, 20 percent owned by Germany's E.ON.