Özel, who is the only commander who did not request retirement, came to the Prime Ministry to meet with Erdoğan late on Friday. Erdoğan met with Özel - the highest-ranking commander who remained in office. Özel was widely expected to become the next head of the military and Koşaner's resignation might speed up the process.
According to Turkish laws, 24 hours must pass for Özel to assume powers of Chief of General Staff.
“The Turkish Armed Forces will continue to do their duty in a spirit of unity,” the office of Prime Minister said in a statement issued after the military's top four commanders quit.
The statement also named Özel as acting Chief of the General Staff. It also said a key Supreme Military Council meeting to decide promotions would go ahead as planned on Monday.
Özel and Erdoğan later went to Çankaya Presidential Palace to have three-way talks with President Abdullah Gül.
By tradition, the head of the land forces replaces the armed forces chief when he retires.
Prime Minister Recep Tayyip Erdoğan earlier ruled out any prospects of tension between the government and the military at a Supreme Military Council (YAŞ) meeting slated for Monday, saying that the decisions to be made at the meeting will be in accordance with the law.
The resignation of so many top commanders for the first time ever in Turkey signals a deep rift with the government, which has been confident in confronting a military that once held sway over Turkish political life. The arrests of high-ranking military officers would once have been unimaginable.
The resignations of Turkey's top generals came hours after a court charged 22 suspects, including several generals and officers, with carrying out an Internet campaign to undermine the government.
The commanders who stepped down decided not to attend a prescheduled reception hosted by the embassy of Turkish Cyprus in a possible move to avoid civilian leaders, NTV television said.
Last August, Turkey witnessed tensions between the military and the Justice and Development Party (AK Party) government when the General Staff attempted to promote a number of generals and admirals standing trial in criminal cases. The government resisted the move and the individuals were eventually not promoted.
YAŞ meets each August to discuss promotions and dismissals within the armed forces. The fact that there are some commanders and military officers who are suspects in ongoing coup cases has led some to speculate that there could be disagreement between the military and the government about the promotion of these individuals. The ruling AK Party had earlier signaled that it would not give the green light for the promotions of these individuals at the YAŞ meeting.
Currently there are 195 suspects, all retired and active duty members of the armed forces, in the ongoing case of Sledgehammer, a suspected coup plan devised at a military gathering in 2003 that allegedly sought to undermine the government in order to lay the groundwork for a military takeover. More senior military personnel have recently been arrested and jailed on charges of links to the subversive coup plan. The government plans to prevent the promotion of 41 Sledgehammer suspects who are active TSK members.
Before this year's YAŞ meeting, Erdoğan, Koşaner and President Abdullah Gül had a summit to prevent a similar crisis from happening. The government and the military agreed that none of the generals that are currently in jail will be promoted.
Gül made a statement on Friday, saying his meeting last week with Erdoğan and Koşaner should not be seen as a glitch in communication. “These are normal things. As the president, I need to know what I am signing when such important decisions are made. I can't blindly sign any document,” he said. The president also said he felt the need to make this statement, speaking to journalists outside a mosque after Friday prayer, because there have been many questions from journalists inquiring if the pre-YAŞ meeting indicated a problem.
Erdoğan on Friday said he wanted no surprises, adding that everything should be done according to the law. “I don't think there will be any tension. The convention will proceed very smoothly. The laws regarding dismissals and promotions are obvious. The laws in this country are functioning normally. What the laws call for will be done.”
Koşaner, who took over as head of the armed forces in August 2010, is regarded as a hardline secularist, but he has kept a lower profile than previous chiefs of the general staff.
Alongside Koşaner, the land forces head Erdal Ceylanoğlu, air forces chief Hasan Aksay and navy commander Uğur Yiğit have also sought retirement.
In a message Koşaner released late on Friday, he said he believes “heroic members” of Turkish armed forces will be successful regardless of any condition in their sacred duty with high discipline, courage and sacrifice.
All the commanders except Koşaner were already set to retire on Monday.
The government made it clear that the appointments and promotions at the upcoming YAŞ meeting will be in line with laws regulating dismissal and promotion, while commanders insisted on upholding long-held traditions the military has adhered to for decades in the appointment and promotion of senior-level commanders.
European Parliament's Turkey rapporteur Dutch Christian Democrat Ria Oomen-Ruijten told Today's Zaman in a phone interview that Turkey is becoming more and more democratic country in which democratic institutions have an oversight on military decisions.
Ruijten declined to comment further on the issue.
In Brussels, a NATO spokeswoman also declined to comment on the resignations. Turkey's military is the second-largest in the 27-member alliance. It has about 1,800 troops as part of NATO's 140,000-strong force in Afghanistan.
"We have confidence in the strength of Turkey's institutions, both democratic and military," US State Department spokesman Mark Toner said in Washington. "This is an internal matter."
It didn't appear that the resignations would immediately affect operational matters on the ground.
The government denies the coup cases are politically motivated and says it is just trying to work to improve democracy.
Erdoğan's ruling party, which won a third term in elections on June 12 in a landslide victory, has said its key goal is to replace a military-era constitution with a more democratic one.
The Turkish military has staged three coups and forced an former prime minister to quit. Coup leaders drew on the support of Turks who saw them as saviors from chaos and corruption, but they were often ruthless.
In a 1960 takeover, the prime minister and key ministers were executed. In a 1980 coup, there were numerous cases of torture, disappearance and extrajudicial killing.
Outside politics, the military enjoys respect and vast economic resources, and is a rite of passage for almost all men who serve as conscripts. It contributes troops in a noncombat role to the NATO-led operation in Afghanistan, and the funerals of soldiers who die in fighting with the outlawed Kurdistan Workers' Party (PKK) members receive heavy media coverage.
The military, however, came under severe criticism after PKK members killed 13 soldiers in a single clash on July 14, prompting the government to order its own investigation and consider deploying special police forces to fight the rebels along with Turkish troops.