Dutch police have arrested three men and a woman after finding explosives in their apartment in the port city of Rotterdam, a police spokeswoman said. "We searched a house in Rotterdam early this morning based on information we had received," the spokeswoman said. She did not disclose what sort of explosives were found nor the identities of those arrested, but said they were being questioned by police. "The explosives are being brought to explode in an open area nearby and then an investigation will start," the spokeswoman said. During the raid, 15 residents from seven neighbouring apartments were evacuated, she said. A Reuters photographer said the area had been cordoned off. The Netherlands' security alert level has been at "substantial" since the bombing attacks in London on July 7 last year, the second highest in a four-stage warning system. The country was shaken by the 2004 murder of a film-maker critical of Islam. Nine young Muslim men were sentenced to jail terms recently for belonging to a terrorist organisation.