Twin car bombs have killed at least 43 people and injured at least 100 in the Turkish town of Reyhanli, near the Syrian border.
Interior Minister Muammer Guler told Turkey's NTV the bombs had gone off near the town hall and post office.
Video showed injured people being carried to safety amid shattered buildings and twisted wrecks of cars.
No group has said it had carried out the attack, but a senior Turkish official suggested Syrian involvement.
"Our thoughts are that their Mukhabarat [the Syrian intelligence agency] and armed organisations are the usual suspects in planning and the carrying out of such devilish plans," Deputy Prime Minister Bulent Arinc said shortly after the bombings.
Later on Saturday another Deputy Prime Minister, Besir Atalay, said initial investigations showed the attackers were linked to Syrian intelligence, NTV reported.
Reyhanli is an entry-point for refugees from the war in Syria and local people attacked cars with Syrian number-plates and Syrian refugees after the attack, according to local media.
The Turkish government has been a key supporter of the Syrian opposition, and has allowed rebels as well as refugees on to its territory.
US Secretary of State John Kerry, Nato Secretary General Anders Fogh Rasmussen and others have condemned the bombing and offered their condolences.
Hours after the initial blasts, reports came in of a third blast in a residential area but the government said it was the fuel tank of a car exploding and not connected to the attacks.
It appears that the first bombs went off 15 minutes apart and video posted on Turkish media shows people running to help victims of the first when there is the sound of a second explosion.
Emergency services looked for possible victims buried under the debris.
"I was sitting in my pharmacy and suddenly we heard a massive explosion," eyewitness Ismail Akin told Reuters news agency.
"When I looked from my window I saw wounded people and dead bodies."
Another witness, Hayrullah Bal, said: "We were a bit far away from the explosions, it suddenly happened and everybody started to run. It was so strong that all the windows shattered."
'All necessary measures'
Turkish Foreign Minister Ahmet Davutoglu said his country would protect itself.
Mr Davutoglu, who was visiting Berlin, said: "There may be those who want to sabotage Turkey's peace, but we will not allow that.
"No-one should attempt to test Turkey's power. Our security forces will take all necessary measures."
He added that the blasts had taken place to deflect attention from efforts to solve the Syrian crisis.
Mr Kerry said: "The United States condemns today's car bombings and we stand with our ally, Turkey.
"This awful news strikes an especially personal note for all of us given how closely we work in partnership with Turkey."
Mr Rasmussen described the bombing in a statement as "despicable" and said Nato stood by Turkey, a member of the alliance.
UK Foreign Secretary William Hague sent a message of solidarity to the people of Turkey.
"Appalling explosions in Reyhanli, Turkey," he wrote on Twitter. "My thoughts are with family & friends of the victims. We stand with the people of Turkey."
The border area of Reyhanli has itself been attacked in recent months.
In February, an explosion near the town killed 17 people and wounded 30.
Five people were killed last October when a mortar round hit the Turkish border town of Akcakale.