At least 30 people were killed when twin explosions hit a rally of hundreds of pro-Kurdish and leftist activists outside Ankara's main train station on Saturday in what the government described as a terrorist attack, weeks ahead of an election.
A Reuters reporter saw bodies covered by flags and banners, including those of the pro-Kurdish opposition Peoples' Democratic Party (HDP), with bloodstains and body parts scattered on the road. The interior ministry said 30 people were killed and 126 wounded.
Witnesses said the two explosions happened seconds apart shortly after 1000 am as crowds gathered for a planned march to protest over a conflict between Turkish security forces and Kurdish militants in the southeast.
There were no claims of responsibility for the attack.
But the NATO member has been in a heightened state of alert since starting a "synchronized war on terror" in July, including air strikes against Islamic State fighters in Syria and PKK bases in northern Iraq. It has also rounded up hundreds of suspected Kurdish and Islamist militants at home.
Footage screened by broadcaster CNN Turk showed a line of young men and women holding hands and dancing, and then flinching as a large explosion flashed behind them, where people were gathered carrying HDP and leftist party banners.
"We are faced with a very big massacre, a vicious, barbarous attack," HDP leader Selahattin Demirtas told reporters.
He drew a parallel with the bombing of an HDP rally in the southeastern city of Diyarbakir on the eve of the last election in June and a suicide bombing blamed on Islamic State in the town of Suruc near the Syrian border in July, which killed 33 mostly young pro-Kurdish activists.
Authorities were investigating claims Saturday's attacks were carried out by suicide bombers, two government officials told Reuters. Prime Minister Ahmet Davutoglu canceled his next three days of election campaigning and was due to hold an emergency meeting with the heads of the police and intelligence agencies and other senior officials, his office said.
The renewed conflict in the southeast has raised questions over how Turkey can hold a free and fair election in violence-hit areas but the government has so far insisted that the vote will go ahead.
President Tayyip Erdogan canceled his engagements to consult with senior security and government officials, while Demirtas and the leader of the main opposition CHP, Kemal Kilicdaroglu, also canceled their programs for Saturday.
"We're ready to come together and work sincerely to finish terror," Kilicdaroglu, whose party is seen as a potential coalition partner for the ruling AK Party after the Nov. 1 election, told reporters in comments broadcast live.
Violence between the state and Kurdistan Workers Party (PKK) militants has flared since July, when Turkey launched air strikes on militant camps in response to what it said were rising attacks on the security forces in the predominantly Kurdish southeast. Hundreds have since died.
Those involved in the march tended to the wounded lying on the ground, as hundreds of stunned people wandered around the streets. Bodies lay in two circles around 20 meters apart where the explosions appeared to have taken place.
"This is a ruthless and barbaric attack on peaceful demonstrators," Council of Europe Secretary General Thorbjorn Jagland said in a statement. "Freedom of assembly and freedom of expression are fundamental pillars of democracy."
Ceasefire had been expected
The attacks come three weeks ahead of an election at which the ruling AK Party is trying to claw back its majority, and at a time of multiple security threats, not only in the southeast but also from Islamic State militants in neighboring Syria and home-grown leftist militants.
In June polls, the AKP lost the overall majority it had held since 2002, partly because of the electoral success of a pro-Kurdish political party, the HDP, which party founder Erdogan accuses of links to the PKK. The HDP denies the accusation.
Saturday's attacks came as hopes mounted that the PKK was about to announce a unilateral ceasefire.
Writing in a Kurdish newspaper this week, senior PKK figure Bese Hozat hinted at a cessation of hostilities as a way of bolstering the chances of the HDP in the upcoming election.
But Deputy Prime Minister Yalcin Akdogan, who is in charge of the Kurdish dossier for the government, on Thursday dismissed any such move as an electoral gambit, pouring cold water on hopes of an imminent end to the violence.
Designated a terrorist group by Turkey, the United States and the European Union, the PKK launched a separatist insurgency in 1984 in which more than 40,000 people have been killed.
The state launched peace talks with the PKK's jailed leader Abdullah Ocalan in 2012 and the latest in a series of ceasefires had been holding until the violence flared again in July.