ISTANBUL — It was a day of mourning for Turkey on Thursday, as a crowd of several thousand people streamed down a central boulevard here, bearing eight coffins draped in Turkish and Palestinian flags, one of them carrying an American citizen.

The outpouring came on the fourth day of a political crisis between Turkey and Israel that has dragged relations between the countries to their lowest point in history. The return of the activists from the flotilla raided by Israel on Monday defused the immediate crisis, but Turkish officials made it clear that it was not over.

“Israel risks losing its most important friend in the region if it doesn’t change its mentality,” said Turkey’s prime minister, Recep Tayyip Erdogan, according to news reports. He called the commando action, in which nine people were killed, “a historic mistake.”

It has been a startling series of events for Turkey, a NATO member and long one of Israel’s closest allies in the Muslim world. But Turkey’s leaders have grown increasingly at odds with Jerusalem over what they believe is an untenable policy in Gaza, a territory run by Hamas, which Israel sees as doctrinally committed to its destruction. Mr. Erdogan has become a sort of folk hero in the Arab world for his open challenges to the government of Prime Minister Benjamin Netanyahu.

The raid served to deepen that divide.

“There is now civilian blood between the two countries,” said Soli Ozel, a professor of international relations at Bilgi University in Istanbul. “The natural arc of relations will have to change.”

President Abdullah Gul said on NTV television, “Turkey will never forgive this attack.”

In Istanbul, where most of the more than 400 Turkish activists were flown early on Thursday morning, traffic clogged the streets as protesters marched next to green Volkswagen vans bearing the coffins, each marked with a name and a city of origin, followed by Gaza in parentheses, denoting solidarity. Marchers wore green headbands, the color of Islam, and peddlers sold Palestinian flags for $3.

“God is great,” mourners chanted in Arabic along with Turkish slogans saying, “Damn Israel” and “An eye for an eye, blood for blood, revenge, revenge.” A woman in a black T-shirt, jeans and sunglasses wore a green headband with the words, “We are all Palestinians now,” echoing statements made after the Sept. 11, 2001, attacks in the United States in which Europeans proclaimed that they were “all Americans now.”

Among the dead was a young man with dual American and Turkish citizenship, Turkish and American officials said. He was identified as Furkan Dogan, a 19-year-old who was born in Troy, N.Y., and lived there as a small child, but later moved back to Turkey. His brother, Mustafa, told the Turkish news media that he was “clean-hearted with a happy face.”

In Washington, Secretary of State Hillary Rodham Clinton said that American officials had spoken to the family to express condolences and offer consular services, and that two other Americans had been wounded in the raid and a subsequent protest, The Associated Press reported. She repeated an earlier call for Israel to “conduct a prompt, impartial, credible and transparent investigation that conforms to international standards.” The United Nations has called for a full international inquiry into the raid, but on Thursday, Israeli officials rejected that demand, news reports said.

The Cihan news agency reported that Mr. Dogan died from bullet wounds to his head and chest, but a spokesman for Turkey’s Foreign Ministry could not confirm that. All nine deaths were caused by bullet wounds, the Turkish authorities said.

“We didn’t expect him to come back like this,” said Mr. Dogan’s brother, who was quoted in Zaman, a Turkish daily newspaper. “However, we were not sorry to hear that he fell like a martyr.”

Martyr is a word usually reserved for Turkish soldiers who die in battle, but has been used repeatedly to describe the dead in the flotilla raid, giving the word a new, Islamist meaning that not all Turks are comfortable with. “They are dragging this county into Middle Eastern quicksand,” said Oray Egin, a columnist with the Turkish daily Aksam. “Gaza is not an emotional issue for me.”

Activists who had returned marched along with the crowd, linking arms with friends and supporters and basking in what people here saw as a heroes’ homecoming. Recep Goker, 51, who had struggled with the soldiers who boarded the ship, was stopped by a tall man in a white pressed shirt, who said he was from Gaza. “You did so good,” the man said. “You are our heroes.”

Mr. Goker said that the Turkish group that led the flotilla was planning another voyage in December, and that he would be part of it. “We will not stop before the embargo is over,” said Mr. Goker, who had a purple bruise on his arm where a plastic bullet had hit him. “We will be the winners, and Israel the loser.”

The Israeli Foreign Ministry said Thursday that all the activists had been deported except for seven who were hospitalized and recovering from injuries, as well as the wife of one of the wounded and two others who had been held up for reasons relating to documentation.