Recep Tayyip Erdogan (left) said conspirators "won't get away with it"
Turkish authorities have detained a further 18 people over an alleged plot to create chaos and trigger a military coup, local media say.
The suspects, 17 of whom are military officers on active duty, were rounded up in a nationwide operation, CNN-Turk and Haber-Turk TV channels said.
Another 49 officers were detained on Monday, 31 of whom have been charged.
The case has increased tension between Turkey's military and its Islamist-rooted ruling party.
News of the arrests came shortly after Turkish Prime Minister Recep Tayyip Erdogan vowed that any conspirators would face justice.
"No-one is above the law, no-one has impunity," he told a gathering of his Justice and Development Party (AKP).
Jonathan Head, BBC News, Istanbul
After a week of high drama at the state security court in Istanbul, the judges have almost completed their assessment of the 49 military suspects detained on Monday.
Thirty-one officers, among them seven navy admirals and four army generals, have been charged with conspiring to provoke a military takeover in the months following the AKP's first election victory in 2002.
That makes this the most ambitious attempt yet to prosecute armed forces personnel in civilian courts. Three other officers, including the general who allegedly masterminded the plot, are still being questioned.
For the once untouchable military the week's events will have been a humbling experience. But it could have been worse; last night the three most senior officers among the 49 were released.
"Those conspiring behind closed doors to trample on the nation's will from now on find themselves facing justice," Mr Erdogan said.
"They should know that they won't get away with it."
The 31 officers charged this week, who include seven navy admirals and four army generals, are accused of conspiring to provoke a military takeover in the months following the AKP's first election victory in 2002.
The case is widely seen as the latest stage in an ongoing power struggle between Turkey's secular nationalist establishment and the governing AKP.
Turkey's military has overthrown or forced the resignation of four governments since 1960 - most recently in 1997 - though the current head of the armed forces has insisted that coups are a thing of the past.
Also on Friday, several suspects were taken to court for questioning, including Gen Cetin Dogan, the former head of Turkey's First Army region.
Some of those detained on Monday have been released. They include the former heads of the air force and navy and a general who were freed on Thursday, having been questioned by prosecutors.
On Thursday, Turkish President Abdullah Gul sought to reassure the country, saying tensions over an alleged military coup plot would be resolved within the "constitution".
He made the statement after meeting the head of the armed forces, Gen Ilker Basbug, along with Mr Erdogan.
HOW 'COUP PLOTS' EMERGED
June 2007: Cache of explosives discovered; ex-soldiers detained
July 2008: 20 arrested, including two ex-generals and a senior journalist, for "planning political disturbances and trying to organise a coup"
July 2008: Governing AK Party narrowly escapes court ban
October 2008: 86 go on trial charged with "Ergenekon" coup plot
July 2009: 56 in dock as second trial opens
Jan 2010: Taraf newspaper reports 2003 "sledgehammer" plot to provoke coup
Feb 2010: More than 40 officers arrested over "sledgehammer"; 20 charged
Reports of the alleged "sledgehammer" plot first surfaced in the liberal Taraf newspaper, which said it had discovered documents detailing plans to bomb two Istanbul mosques and provoke Greece into shooting down a Turkish plane over the Aegean Sea.
The army has said the scenarios were discussed but only as part of a planning exercise at a military seminar.
The alleged plot is similar, and possibly linked, to the reported Ergenekon conspiracy, in which military figures and staunch secularists allegedly planned to foment unrest, leading to a coup.
Scores of people, including military officers, journalists and academics, are on trial in connection with that case.
Critics believe the Ergenekon and sledgehammer investigations are simply attempts to silence the government's political and military opponents.
The AKP has its roots in political Islam, and is accused by some nationalists of having secret plans to turn staunchly secular Turkey into an Islamic state.
The government rejects those claims, saying its intention is to modernise Turkey and move it closer to EU membership.
"The process underway is painstaking, but it is for the benefit of the people, today's developments are setting free the consciousness of the people," Mr Erdogan said.