Turkey's top military commanders, self-appointed guardians of its staunchly secular system, held an emergency meeting late on Tuesday and warned in a statement of a "serious situation."

Prime Minister Tayyip Erdogan subsequently met overnight with his deputy and the interior and justice ministers after a visit to Spain. Details of the talks were not disclosed.

Fears of the situation escalating are driven by a history of military intervention in politics in NATO-member Turkey although the military has said the days of military coups are over.

The army has unseated four governments in the last 50 years, however similar moves are no longer seen as feasible. The power of the military has been eroded in recent years by reforms designed to facilitate Turkey's EU accession.

The tensions were triggered by a dramatic police swoop on Monday that detained 50 officers in an unprecedented operation.

The seven officers charged overnight are four admirals, two retired and two serving; a retired brigadier-general and two retired colonels, state-run Anatolian news agency said.

They have been jailed awaiting trial. Pending a formal indictment, they are accused of membership in a terrorist group and attempting to overthrow the government by force.

The increased political tension hit investor confidence. The lira weakened to a seven-month low against the dollar, while shares fell 2.1 percent and bond yields rose. The Istanbul stock index has fallen by 5.6 percent this week.

The ruling AK Party, which secularists accuse of trying to undermine Turkey's secular system through a gradual Islamization of society, is also embroiled in a deepening dispute with the secularist judiciary. The party denies the charges.

Retired commanders of the air force and navy and a former deputy armed forces chief are among those detained at police headquarters, awaiting questioning by prosecutors.

A further group of serving and retired officers was brought to a courthouse on the banks of Istanbul's Bosphorus Strait for questioning by prosecutors on Wednesday morning.

Six officers were released from custody on Tuesday after questioning. It was not clear if they would face charges.

The army leadership has said previously that probes of a series of alleged coup plots was hurting morale in the ranks.

In a characteristically veiled and brief statement on its web site on Tuesday, the General Staff said its top commanders had met to "assess the serious situation that has arisen."

"What do you mean? Are you going to carry out a coup?" said a headline on the military statement in Taraf newspaper, which has repeatedly broken news of alleged coup plots.

The current investigation into the so-called "Sledgehammer" plan, allegedly drawn up in 2003, was triggered by a report in Taraf last month. The military has said the plan was just a scenario drawn up for an army seminar.

Taraf published plans it said showed officers had plotted to provoke Greek fighter jets into shooting down a Turkish military jet, and to plant bombs in Istanbul mosques.

Retired military officers are among around 200 people indicted over separate plots by a far-right group known as Ergenekon. Critics say that trial is being used to target political opponents, an accusation the government rejects.