An explosion in the early hours of the morning damaged Amsterdam's court complex.

Police were investigating the cause of the 2.30am blast, which damaged several floors at one of the towers that make up the courthouse. No one was injured.

A government minister swiftly condemned the blast as an attack on the Dutch justice system.

"It is an attack on the judicial system," Security and Justice Minister Ivo Opstelten told national broadcaster NOS. "We have to pull out all the stops to catch the perpetrators."

The damage was to one of several office blocks in the complex. Opstelten said that it had not prevented any trials going ahead.

Television images showed workers removing shards of shattered glass from window frames and a small hole in one of the concrete walls.

NOS reported that it appeared that a projectile had been fired at the building and hit a stairwell.

"We have to find the attackers and the motive," Opstelten said. "It is an unbelievable level of violence."

Police made no immediate arrests.