The attack, which raised questions about the Turkish government’s ability to protect its citizens, occurred two days after the United States Embassy warned of a potential terrorist plot to attack government buildings and residences in Ankara.
Turkey once sought to contain the chaos unfolding across the Middle East, but is now increasingly being sucked into the violence. Devastating bombings, some linked to the extremists of the Islamic State and others to Kurdish militants who have been carrying out a long insurgency against the Turkish government, have struck gatherings of activists, Turkish military targets and a landmark tourist site in Istanbul. But the attack on Sunday seemed to suggest a shift in tactics, with the targeting of a large gathering of civilians in a major transportation hub.
At least 34 people were killed and 125 wounded in the attack, Health Minister Mehmet Muezzinoglu said. There were no immediate claims of responsibility. The Turkish authorities said an investigation was underway.
In a written statement, President Recep Tayyip Erdogan said terrorist organizations were singling out civilians because the groups were losing their struggle against Turkish security forces.
Three weeks earlier, a bombing on a military convoy in Ankara killed 28 people. A militant group based in Turkey called the Kurdistan Freedom Falcons claimed responsibility for that attack, identifying the bomber as a 26-year-old Turkish citizen. The Turkish government blamed a Syrian Kurdish militia that is supported by the United States in the fight against the Islamic State in Syria.
That bombing was believed to be a response to counterinsurgency operations in the predominately Kurdish southeast. Turkey has been shelling positions held in northern Syria by Kurdish militias that it deems to be extensions of the Kurdistan Workers’ Party. The party, known as the P.K.K., has been fighting for autonomy for more than three decades.
But while that attack struck Turkish security forces, the attack on Sunday appeared intended to kill civilians, stoking fears of a spillover of violence to metropolitan areas.
Photographs of the blast area on Sunday showed several buses and vehicles on fire, as well as shattered glass from nearby shop windows.
“It looks and sounds larger than the attack last month,” said Mehmet Arabaci, an Ankara resident who took photographs of the scene after hearing the explosion.
Almost immediately, the Turkish authorities, as they had after other attacks, imposed a ban on local news media coverage of the bombing. And a short while later, a court in Ankara issued an order blocking access to social media in an effort to prevent the dissemination of photographs from the bombing scene.
Mr. Arabaci said Ankara residents had been avoiding crowded areas after three major attacks in six months in the city killed more than 150 people. “We don’t know when and where there will be another attack, but it’s apparent now that they can’t be prevented, and everyone is on edge,” he said.
Kizilay Square, the scene of the attack on Sunday, is one of Ankara’s most popular commercial and entertainment hubs. Security analysts said the death toll would have been far higher if the attack had been carried out at the same time on a weekday, rather than on a weekend.
In October, the capital was rocked by what officials called the deadliest terrorist attack in the country’s modern history. Two suicide bombers believed to have been affiliated with the Islamic State struck a peace rally, killing more than 100 people, mainly Kurds.
A pro-Kurdish party that won representation in Parliament for the first time last year condemned the attack on Sunday, saying the party “shared the huge pain” felt by other citizens.
The statement was significant because the government has accused the party of having ties to the P.K.K., which the party’s leaders vehemently deny. Last week, Prime Minister Ahmet Davutoglu applied to Parliament to lift the immunity of the senior party members in order to prosecute them for belonging to a terrorist organization.
On Sunday, Mr. Erdogan said Turkey would continue to assert its right of self-defense against terrorist threats, and he vowed to carry on the fight against terrorist groups. A statement from the White House said the United States condemned the bombing and would continue to stand with Turkey in the fight against terrorism.