Turkish President Recep Tayyip Erdogan has denounced as "heinous" a bombing in Ankara that left dozens killed and more than 180 people injured.
Two blasts struck an area outside Ankara's main train station on October 10, killing 86 people and injuring another 186, according to Turkish authorities.
Health Minister Mehmet Muezinoglu said 62 people died at the scene while 24 others died after being taken to the hospital.
Turkey declared a three-day official mourning for the victims as well as for people killed in terror attacks since July.
Later on the day, hundreds of protesters gathered in Istanbul, chanting antigovernment slogans and blaming the government for the Ankara bomb attacks.
Riot police with two water cannons watched on as the crowd tried to march towards Istanbul's central Taksim Square.
Police have previously prevented demonstrators from trying to reach Taksim Square, the epicenter of weeks of violent antigovernment protests during the summer of 2013.
The government describes the incident as a "terrorist attack" and says it is investigating the possibility that it was a suicide bombing.
"I strongly condemn this heinous attack on our unity and our country's peace," Erdogan said in a statement posted on the presidential website.
Prime Minister Ahmet Davutoglu said there are strong signs that the blasts were suicide bombings.
The explosions occurred at an area where people were gathering for a planned "peace march" to push for a settlement of the conflict between the government and Kurdish militants in the southeastern part of the country.
"There was a demonstration," one witness told Reuters. "I was walking next to a stage rally truck. Right here, behind two banners, an explosion went off. We laid on the ground. The second bomb went off there. There were two bombs, but the one that went off here was a very strong one."
The march -- which has been cancelled -- was organized by pro-Kurdish and leftist political parties. Police reportedly had to fire warning shots into the air to disperse angry crowds at the scene of the blasts.
The two explosions occurred several minutes apart, according to witnesses.
The White House condemned what it called a "horrific terrorist" attack.
The U.S. Embassy in Ankara issued a statement on Twitter condemning the attack and saying, "All of us must stand united against terror."
Russian President Vladimir Putin telegraphed his condolences to Erdogan, the Kremlin press office reported. Putin expressed Russia's willingness "to work together with the Turkish authorities as closely as possible in opposing the terrorist threat," the statement said.
Russia's Foreign Ministry said the attack "confirms the need to consolidate the efforts of all regional and extraregional partners in combatting terror."
EU foreign policy chief Fredrica Mogherini said Turkey "must stand united" in the wake of the attack.
Meanwhile, the Kurdistan Workers Party (PKK) militant group declared a halt to guerrilla activities in Turkey.
The group made the announcement on October 10, saying its fighters would avoid actions that could prevent a "fair and just" election being held in Turkey on November 1.
The PKK's statement did not mention the bombing in Ankara.
The announcement had been anticipated earlier in the week when a top PKK commander wrote a newspaper column floating the idea.
Turkish Deputy Prime Minister Yalcin Akdogan dismissed the idea earlier this week as a mere "tactic" to boost support for the pro-Kurdish People's Democratic Party (HDP).
Akdogan repeated the government's call for PKK militants to lay down their arms and leave the country.