He said he heard a huge boom which shook the ceiling.

Participants were later told that a boiler had exploded, he added. The Forum's main programme was not disrupted.

Swiss prosecutors said they were investigating the explosion, implying that there might be a criminal motive, but they declined to give further details.

A Reuters photographer said a few police were patrolling outside the building and he could see a broken window but no other damage.

A group calling itself Revolutionary Perspective said in a statement on an activist website it had targeted the luxury Posthotel with a firebomb and said Swiss ministers and representatives of top bank UBS were staying there.

"Our fight against the dictatorship of capital is focused on the social alternative to capitalism: communism," the group said in the statement.

A spokesman for the World Economic Forum (WEF) said police had made two bomb sweeps of the hotel after a threat posted online. The WEF said in a statement that the blast had been caused by a small firework at the back entrance of the hotel.

The luxury Posthotel is in the heart of Davos village, a few hundred metres from the WEF congress centre where hundreds of company heads, central bankers and politicians are meeting.

On Wednesday (January 27), Swiss police evacuated a building and removed a suspicious object in the town St. Gallen, 80 km north of Davos, after a threat from the same group, saying they wanted to target the opening of the WEF meeting.