Turkish Prime Minister Recep Tayyip Erdogan said two car bombs which killed at least 20 people in Istanbul were aimed at destabilising the country.
The blast struck worshippers at prayer
Mr Erdogan cut short a visit to northern Cyprus in the wake of the bombings at the city's synagogues.
At least 20 people were killed, most of them at Istanbul's largest synagogue, the Neve Shalom.
At least six of those who died were Jewish, according to members of the local community.
A radical Turkish Islamist group, known as the Great Eastern Islamic Raiders' Front, said it was responsible, but the claim is being treated with scepticism.
The Turkish Foreign Minister, Abdullah Gul, said he believed suicide bombers were behind the blasts. "It is clear that this is a terrorist event with international links," he said.
Israel has denounced the blasts as "criminal terror attacks".
Israeli Foreign Minister Sylvan Shalom is scheduled to visit Istanbul on Sunday.
He will meet Istanbul's Jewish community, convey Israel's condolences after the attacks and hold talks with his Mr Gul.
US President George W Bush condemned the attacks in Turkey where, he said, Muslims, Christians and Jews had flourished together for centuries.
Mr Erdogan was forced to return from Turkish northern Cyprus, which is celebrating 20 years of self-declared independence.
"We were here today to celebrate ... an anniversary. But
we all got this very upsetting news," he said.
"This is a bomb aimed at stability and peace in the
Turkish republic. We're definitely condemning this, no
matter which group planned it," he said.
The car bombs exploded outside the synagogues at about 1000 (0700 GMT) as worshippers were holding Sabbath morning prayers.
The facade of Neve Shalom collapsed and the second synagogue, the Beth Israel, about five kilometres (3 miles) away in Sisli district, was also severely damaged.
"The blast went off in the middle of the prayer," Turkish Chief Rabbi Yitzhak Haleva said of the explosion at Neve Shalom.
"All the windows were shattered and I suddenly found myself surrounded by thick smoke."
Eyewitness Enver Eker, who was near Neve Shalom went the bomb went off, said: "There was huge panic, glass exploding and metal pieces all over the place."
Istanbul health officials said 257 people were wounded in the attacks.
Television footage showed wounded people staggering in the street, some with bloodied or charred faces.
Many of the dead were passers-by or local shopkeepers. Some tradespeople in the area of the synagogue are Jewish.
No-one died inside Neve Shalom, although the synagogue was at the time packed with people attending a bar-mitzvah coming-of-age ceremony.
At least three Jews are said to have been killed at the Beth Israel synagogue, as well as a policeman.
The powerful blasts turned cars into mangled wrecks and shattered windows across a wide area.
"It was like a war zone," said eyewitness Sadettin Gul.
The Neve Shalom synagogue was the scene of an earlier attack in 1986, when Palestinian gunmen killed 22 worshippers and wounded six others during a Sabbath service.
A caller claiming to be from a radical Turkish group, Great Eastern Islamic Raiders' Front - also known as IBDA/C - said it carried out the attack, Anatolia reported.
The caller was quoted as saying: "The reason [for the attacks] is to stop the oppression of the Muslims... Our acts will continue."
The blast left a crater outside the Neve Shalom synagogue
The IBDA/C carried out a series of attacks around Istanbul in the 1990s, but has since been largely dismantled by Turkish security forces.
It has never been involved in an incident on this scale.
Turkish officials are sceptical about the claim and are examining whether foreign groups were behind the attacks.
Correspondents say they were highly professional and well co-ordinated, suggesting involvement of the al-Qaeda network.
Istanbul has a small Jewish community that generally has very good relationships with other Turks.