The bombing increased fears that Syria's civil war was dragging in neighbouring states despite renewed diplomatic moves towards ending two years of fighting in which more than 70,000 people have been killed.
The bombs ripped into crowded streets near Reyhanli's shopping district in the early afternoon, scattering concrete blocks and smashing cars in the town in Turkey's southern Hatay province, home to thousands of Syrian refugees.
Restaurants and cafes were destroyed and body parts were strewn across the streets. The damage went at least three blocks deep from the site of the blasts.
President Bashar al-Assad's administration was the "usual suspect" in the attacks, Turkish Deputy Prime Minister Bulent Arinc said.
"We know that the people taking refuge in Hatay have become targets for the Syrian regime," Arinc said in comments broadcast on Turkish television. "We think of them as the usual suspects when it comes to planning such a horrific attack."
Another deputy prime minister, Besir Atalay, was quoted by NTV as saying initial findings suggested the attackers came from inside Turkey, but had links to Syria's intelligence agency.
There was no immediate claim of responsibility. Nor was there any comment from Damascus.
The United States strongly condemned the attacks and vowed support in identifying those responsible, while NATO Secretary-General Anders Fogh Rasmussen and French Foreign Minister Laurent Fabius voiced "full solidarity" with Turkey.
NATO member Turkey supports the uprising against Assad and violence has crossed the border before, but not on the same scale. The bombings were the bloodiest incident on Turkish soil since Syria's conflict began more than two years ago.
Turkey is far from alone in fearing the impact of Syria's war, which is already helping inflame the Middle East's tangle of sectarian, religious and nationalist struggles.
Turkish Foreign Minister Ahmet Davutoglu said it was no coincidence the bombings came as diplomatic moves to end the Syrian conflict intensify.
"There may be those who want to sabotage Turkey's peace, but we will not allow that," Davutoglu told reporters during a trip to Berlin. "No one should attempt to test Turkey's power."
Prospects appeared to improve this week for diplomacy to try to end the civil war, now in its third year, after Moscow and Washington announced a joint effort to bring government and rebels to an international conference.
But a Russian official said on Saturday that there was already disagreement over who would take part and he doubted whether a meeting could happen this month.
As well as disputes over who would represent the rebels and government at any talks, there have also been questions over possible participation by Assad's Shi'ite ally Iran. The rebels are backed by the largely Sunni Gulf states.
Diplomats in New York said the Syria meeting would likely slip into June and it was unclear who would participate.
DEATH TOLL MAY RISE
In Reyhanli, smoke poured from charred ruins after the blasts outside administrative buildings.
"My children were so scared because it reminded them of the bombings when we were in Aleppo. God help us," said one refugee from the northern Syrian city, a mother of three who gave her name as Kolsum.
Atalay said 43 people had been killed, while Prime Minister Tayyip Erdogan warned the toll could rise. Officials said more than 100 people were injured, many of them critically.
Erdogan said the bombings might have been related to Turkey's own peace process with Kurdish militants of the Kurdistan Workers Party (PKK), who began a withdrawal this week to end a three decade conflict.
But the PKK's main area of operation was hundreds of kilometres further east and Erdogan said the blasts could also have been aimed at provoking sensitivities in a region that is home to so many Syrian refugees.
Turkey is sheltering more than 300,000 Syrians, most of them in camps along the 900-km (560-mile) frontier, and is struggling to keep up with the influx.
Protests erupted in Reyhanli after the blasts, with some locals blaming Syrian there for bringing violence over the frontier and smashing their car windows, while others railed against Turkey's foreign policy, chanting for Erdogan to resign.
The main opposition Syrian National Coalition said the attacks were a failed attempt to "destroy the brotherhood" between Syrians and Turks and were intended as a punishment for Turkey's support of the uprising.
Erdogan said this week Turkey would support a U.S.-enforced no-fly zone in Syria and warned that Damascus crossed President Barack Obama's "red line" on chemical weapons use long ago.
A no-fly zone to prohibit Syrian military aircraft from hitting rebel targets has been mentioned by American lawmakers as one option the United States could use to pressure Assad.
Erdogan is due to meet Obama in Washington on May 16.
Violence also crossed the border in February, when a minibus blew up at a border crossing near Reyhanli, killing 14 people.
The Syrian opposition said one of its delegations appeared to have been the target of that attack, but there has been no confirmation of this from the Turkish authorities.
In October, five Turkish civilians were killed in Akcakale when a mortar bomb fired from Syria landed on their house, prompting Turkey to fire back across the frontier.
