Dutch police arrested seven Islamic extremists suspected of plotting attacks against politicians and government buildings in a series of raids across the Netherlands today.

Government ministries in The Hague were sealed off and gunshots heard in the capital as anti-terrorist officers swooped to arrest the six men and one woman.

One of the men held was acquitted earlier this year on charges of planning terrorist attacks and all were said to belong to the Hofstad group, one of whose members is serving a life term for the murder of filmmaker Theo van Gogh.

Johan Remkes, the Interior Minister, said that the raids had elimated an "acute threat".

"The intelligence services have established that despite earlier arrests this network has continued its activities. The group has grown in size in the last year and does not only consist of young men but also of young women," he said.

"The arrests do not mean that all threat of a possible attack have disappeared but an acute threat was eliminated."

The national prosecutor’s office said the main suspect, Samir Azzouz, 19, was allegedly trying to buy automatic weapons and explosives "probably to carry out an attack with others on several politicians and government buildings".

The Dutch national anti-terrorism coordinator said that the arrests had led to security measures being stepped up at high-profile targets in the country, adding that the terrorist threat was "substantial".

Around two dozen police officers in riot gear closed entrances leading to both houses of parliament and the government’s information service as the raids were carried out in The Hague, Amsterdam and nearby Almere. Prosecutors refused to identify those arrested, with the exception of Azzouz.

They said there were another five men aged between 18-30 and one 24-year-old woman. The suspects, who are all Dutch citizens, are due to appear in court on Monday.

Public radio said that raids had been carried out on a school in The Hague and that eyewitnesses reported hearing gunfire in the largely immigrant Schilderswijk neighbourhood of the Dutch capital. Police refused to confirm the reports.

"I walked around the corner and saw someone waving a gun shooting in the street," an unidentified witness told the NOS broadcaster.

The Dutch media reported yesterday that renewed threats had been made against two members of parliament, Ayaan Hirsi Ali and Geert Wilders, who are both outspoken critics of Islamic extremism. The two went into hiding for several months last year after the murder of filmmaker Theo van Gogh in Amsterdam.

Dutch authorities raised a national security alert last July after they arrested Azzouz and found machinegun cartridges, a bullet-proof vest, two mock explosive devices, a silencer, maps and sketches of prominent buildings in his home. But a Rotterdam court ruled in April that there was not enough evidence to convict Azzouz for plotting bomb attacks.

He was sentenced to three months imprisonment for illegal possession of weapons, but was freed on the day of the hearing because of time already spent in custody.

Prosecutors have separately linked Azzouz to a militant Islamist network that is suspected of plots to kill leading politicians critical of Islam and of ties to the man charged with the murder of Mr van Gogh last November.

Twelve members of the so-called "Hofstad" group were detained after van Gogh was killed, and they face trial for membership of a criminal organisation and planning to kill prominent politicians.

The country’s security alert has been at "substantial" since the bombing attacks in London on July 7. This is the second highest threat in the Netherlands’ four-stage warning system.

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