(Updates number of those arrested)

By Selcuk Gokoluk

ANKARA, July 5 (Reuters) - Eight out of 21 prominent Turks detained in a police investigation into coup allegations have been formally arrested and two senior former generals have been taken to court, Turkish media said on Saturday.

The chairman of Ankara's Chamber of Commerce, Sinan Aygun, was among the eight, the state-run Anatolian news agency said. Aygun is a vocal secularist critic of the ruling AK Party, which faces court charges of trying to introduce Islamic rule.

Eleven detainees, including the Ankara representative of staunchly secularist Cumhuriyet newspaper, Mustafa Balbay, and a retired admiral, have been released but some were barred from leaving the country while the case continued.

"I was detained on suspicion of being member of a terror organisation, but I feel like I was wounded by terror," Balbay told reporters after his release.

All of the 21 people detained on Tuesday were known critics of the AK Party government. They were detained on Tuesday, hours before the first hearing in a case in which the AK Party faces possible closure on charges of trying to introduce Islamic rule.

The party dismisses the charges as politically motivated and points to its pro-business, pro-reform track record in office.

Former head of the paramilitary gendarmerie Sener Eruygur and former first army commander Hursit Tolon were sent to court in Istanbul on Saturday, Anatolian said. The prosecutor sought the arrest of the former generals but the court has yet to rule.

"SECRET PLAN"

Turkey, a predominantly Muslim but officially secular country seeking to join the European Union, has had four military coups in the last 50 years. No top generals have been arrested for political reasons during that time. Turkish media said those detained were suspected of being members of a shadowy, ultra-nationalist, hardline secularist group known as Ergenekon, which was already under investigation.

A secret plan, including launching illegal protests on July 7 across 40 provinces, assassinations and clashes with security forces, was seized during Tuesday's swoop, media said.

The prosecutor was not immediately available and Istanbul police declined comment.

Dozens of people had already been detained for suspected links to the Ergenekon group, including retired army officers.

Opponents of the government have questioned the coup allegations, pointing to the fact that no indictment has been released despite a 13-month long investigation.

The military -- which has repeatedly criticised the government and considers itself the guardian of Turkey's secular system -- has denied any links to the Ergenekon group.

The high-profile arrests and start of the case to close the party hit Turkish stock prices and the lira currency on Tuesday. The markets later rebounded thanks to foreign cash inflows. (Editing by Philippa Fletcher)

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