ISTANBUL, July 5, 2008 (AFP) - Four people, including a retired army officer and a top businessman, were remanded in custody Saturday by a Turkish court in connection with an alleged plot to overthrow the government, the Anatolia news agency reported.

It was not immediately clear what charges had been brought against the suspects, who included Sinan Aygun, the head of the Ankara Chamber of Trade (ATO), and Atilla Ugur, a retired colonel, the report said.

The four were among 21 suspects rounded up by police earlier this week in dawn raids in five cities as part of a controversial probe into a shadowy network called Ergenekon, that allegedly aimed to discredit and oust the Islamist-rooted government.

Retired four-star generals Hursit Tolon and Sener Eruygur as well as Mustafa Balbay, the Ankara representative of the secular, anti-government daily Cumhuriyet, were among those taken into custody in the raids.

The three men, all of them vocal critics of the government, have yet to be brought before a court to be formally charged or released.

The detentions came at a time when the ruling Justice and Development Party (AKP) was defending itself in court against charges that it is seeking to undermine the country's secular order and should be banned.

The Ergenekon investigation was launched in June last year after the discovery of explosives in an Istanbul house, and courts have so far remanded 54 suspects in custody, including the four on Saturday.

But the investigation has come under strong criticism because prosecutors are yet to issue an indictment and detail the charges against the suspects.

Critics say the ruling party is deliberately expanding and prolonging the affair to intimidate and silence opponents.