Turkey's prime minister says the four suicide bombers who staged attacks over the past week against British interests and Jewish synagogues in Istanbul were Turkish citizens. Anti-terrorist police are convinced that the men had links to foreign terrorists.

Speaking at the funeral of two policemen who died in Thursday's bombings at the British consulate general and the Istanbul headquarters of the London-based HSBC bank, Turkish Prime Minister Recep Tayyip Erdogan said Turkish nationals carried out those attacks, as well as last Saturday's bombing of two synagogues.

Mr. Erdogan said Turkey should be ashamed that four of its own citizens perpetrated the attacks, which have caused widespread revulsion among Turks.

Mr. Erdogan said the death toll from Thursday's bombings had risen to 30. Twenty five people died in the attacks on the synagogues. Although the bombings were aimed at Istanbul's 500-year-old Jewish community in one instance and British targets in the other, most of the victims were Turkish Muslims.

The normally reliable Turkish newspaper Hurriyet reported Saturday that 18 people have been arrested in connection with Thursday's bombings. But police had no official comment on the report. The government has admitted that it has made several arrests, but has given no further details. Six people have been arrested in connection with the synagogue bombings.

Responsibility for all four attacks was claimed by two groups, one identifying itself as part of the al-Qaida terrorist network. The other was a Turkish group that said it had acted together with al-Qaida.

While expressing his shame over the involvement of Turks in the attacks, Mr. Erdogan said investigators are certain that the bombers had ties to foreign groups.

Turkish news media report that anti-terrorist police are concentrating their search on Turkish Islamic radicals who fought in Afghanistan, Bosnia and Chechnya, where they may have made contact with other extremists. The reports say most of those Turks are now back in Turkey, and may be linked to home-grown militant groups.

Across Turkey Saturday, thousands of people took to the streets to protest the bombings in Istanbul at rallies organized by trade unions and civic organizations. Although some leftists noisily blamed the United States for the attacks, saying the U.S.-led war in Iraq and Turkey's close ties to Washington had put their country in the terrorist firing line, most of the demonstrators expressed their outrage over the bombings.

Professor Solmaz Ayarslan, the director of a foreign language school in Istanbul, says Turks will not be demoralized by the atrocities, and will stand up to terrorism.

"Life in Istanbul is not going to be interrupted because of these activities," he said. "I can assure you of that."

They may strike a chord of defiance, but the people of Istanbul are also feeling apprehensive. With the United States and Britain warning that Turkey may face new attacks, security has been visibly tightened at potential targets like foreign consulates, schools and shopping malls. Officials say the increased police presence is also intended to calm public nervousness.