Mugabe signed the new media law in March

The journalists argue that the legislation, which was introduced in March, violates freedom of expression.

A total of eight journalists have so far been charged under the law, which the government insists is necessary to tackle a collapse in journalistic standards.

One of those behind the challenge has been charged under the media law, and could face a penalty of up to two years in jail.

He is Andrew Meldrum, a US citizen writing for Britain's Guardian newspaper.

The other two, Peta Thornycroft and Jan Raath, are also correspondents for British papers - the Daily Telegraph and The Times respectively.

Falsehood

The BBC's Grant Ferrett, a former Harare correspondent, says the ruling Zanu-PF party has made plain its hostility to much of the local and foreign media in recent months.

Government supporters have attacked journalists working for the privately-owned media and ministers have denounced some foreign correspondents as terrorists.

The independent press is feeling pressured

Last week Mr Meldrum and two reporters working for Zimbabwe's Daily News, Lloyd Mudiwa and Collin Chiwanza, were briefly detained in connection with an unsubstantiated story about an alleged political murder.

The Daily News, which originally ran the story, later withdrew it and apologised to Zanu-PF.

Mr Meldrum and Mr Mudiwa have been charged with publishing a falsehood. They are due to appear in court again later this month.

Mr Chiwanza has been cleared.

The editor of the Daily News - Zimbabwe's only privately-owned newspaper - Geoff Nyarota has been arrested several times since the paper was launched in 1999.

Crackdown

The Access to Information and Protection of Privacy bill was the subject of intense international criticism in the run-up to Zimbabwe's presidential election in March.

The Daily News editor has been arrested five times

Among the new restrictions it imposes are a licensing system for journalists and a ban on non-resident foreign correspondents being based in the country.

The government delayed implementing the legislation until after the poll, which saw President Robert Mugabe gaining another term in office in the face of opposition allegations of vote-rigging.

The Information Minister, Jonathan Moyo, denies that the new law amounts to a crackdown on the media, instead describing it as a crackdown on crime.