Two senior security officials have said initial findings suggested the Kurdistan Workers Party (PKK) militant group was responsible.
The death toll in a car bomb attack in the Turkish capital Ankara has risen to 37 people, Health Minister Mehmet Muezzinoglu said on Monday, adding that 71 people were still being treated in hospital.
Of those in hospital, 15 were in serious condition, he told reporters.
Sunday's bombing was the second such attack in the administrative heart of the city in under a month and two senior security officials have said initial findings suggested the Kurdistan Workers Party (PKK) militant group was responsible.
The bomb was the second such attack in the administrative heart of the city in less than a month.
The blast, which could be heard several kilometres away, sent burning debris showering down over an area a few hundred metres from the Justice and Interior Ministries, a top courthouse, and the former office of the prime minister.
Police helicopters hovered overhead as a large cloud of smoke rose over the city centre.
"A total of 27 of our citizens were killed when a car exploded at Kizilay's Guven Park, and close to 75 of our wounded citizens were taken to various hospitals for treatment," the Ankaragovernor's office said in a statement.
Two security officials later said the death toll had risen to 34 people.
One senior security official told Reuters initial findings suggested the attack had been carried out by the Kurdistan Workers Party (PKK) or an affiliated militant group, but there was no immediate claim of responsibility. A second official said gunfire was heard after the blast.
Another official said the car used in the attack was a BMW which had been driven from Viransehir, a town in the largely Kurdish southeast. The PKK and the Kurdistan Freedom Hawks (TAK) appeared to be responsible, he said.
TAK claimed responsibility for the previous car bombing, just a few blocks away on Feb. 17. That bombing killed 29 people, most of them soldiers, near the military headquarters, parliament and other key government institutions.
Prime Minister Ahmet Davutoglu held an emergency meeting with the interior minister, the head of the intelligence agency and police and security chiefs, officials said. President Tayyip Erdogan spoke by phone with the interior minister.
The pro-Kurdish opposition HDP, parliament's third largest party, which Erdogan accuses of being an extension of the PKK, condemned what it described as a "savage attack".
An Ankara court ordered a ban on access to Facebook, Twitter and other sites in Turkey after images from the bombing were shared on social media, broadcasters CNN Turk and NTV reported.
State broadcaster TRT said the car had exploded at a major transport hub, hitting a bus carrying some 20 people near the central Guven Park and Kizilay Square. It said the area was crowded when the explosion happened at 6:43 p.m. (1643 GMT).
European leaders condemned the bombing. British Prime Minister David Cameron said he was "appalled". French Foreign Minister Jean-Marc Ayrault described it as a "cowardly attack".
NATO member Turkey faces multiple security threats. As part of a U.S.-led coalition, it is fighting Islamic State in neighbouring Syria and Iraq. It is also battling PKK militants in its southeast, where a 2 1/2-year ceasefire collapsed last July, triggering the worst violence since the 1990s.
Turkey sees the unrest in its largely Kurdish southeast as deeply linked to events in northern Syria, where the Kurdish YPG militia has been seizing territory as it fights both Islamic State and rebels battling President Bashar al-Assad.
Ankara fears those gains will stoke separatist ambitions among its own Kurds and has long argued that the YPG and PKK have close ideological and operational ties.
In its armed campaign in Turkey, the PKK has historically struck directly at the security forces and says that it does not target civilians. A claim of responsibility for Sunday's bombing would indicate a major tactical shift.
The Kurdistan Freedom Hawks, or TAK, which claimed responsibility for the Feb. 17 attack, was once affiliated with PKK but says it has split from the group.
The U.S. embassy issued a warning on March 11 that there was information regarding a potential attack on government buildings in the Bahcelievler area of Ankara, several kilometres away from the site of Sunday's blast.
Islamic State militants have carried out at least four bomb attacks on Turkey since June 2015, including a suicide bombing which killed 10 German tourists in the historic heart of Istanbul in January. Local jihadist groups and leftist radicals have also staged attacks in the NATO member country in the past.