Mr Wilders is the leader of the Dutch Freedom Party

It is the second-highest alert level, although the justice ministry said "there is no concrete evidence" that the country faced possible attacks.

The move comes as far-right Dutch MP Geert Wilders prepares to air his film, which has already angered Muslims.

Mr Wilders has said the film is about the Koran, but gave few details.

Defiance

He has revealed that his 15-minute film is entitled Fitna, an Arabic word used to describe strife or discord, usually religious.

The project has already been condemned by several Muslim countries, including Iran and Pakistan.

The lawmaker has said his work will show how the Koran is "an inspiration for intolerance, murder and terror".

According to a Dutch daily which has seen some of the footage, the film has the Koran opening.

Inside the pages of the book are shown images of atrocities in Muslim countries that the film-maker thinks are inspired by verses of the Koran.

Last month, Mr Wilders said he expected that his work would be shown in the Netherlands in March and also released on the internet.

He said he was determined to release the film, despite government warnings that this would damage Dutch political and economic interests.

Van Gogh murder

In the past, Mr Wilders - who leads the Freedom Party - has called for the Koran to be banned and likened it to Adolf Hitler's Mein Kampf.

He has described Dutch culture as superior to what he says is a retarded Islamic culture and believes immigrants must assimilate by getting rid of what he calls the intolerant and fascist parts of the Koran.

Mr Wilders has had police protection since Dutch director Theo Van Gogh was killed by a radical Islamist in 2004.

Mr Van Gogh's film Submission included verses from the Koran shown against a naked female body.

As well as the killing of Mr Van Gogh, Dutch politicians are mindful of the widespread protests by Muslims that followed the publication of cartoons satirising the Prophet Muhammad in newspapers in Denmark and other European countries in 2006.