By Daren Butler

ISTANBUL, Nov 22 (Reuters) - Several thousand Turks gathered in Istanbul and other cities on Saturday to condemn this week's suicide bombings, with many protesting against what they see as the root cause of the attacks -- the United States.

Police in riot gear looked on in Turkey's business capital as unions, political parties and community groups peacefully joined those affected by the blasts to share their sadness and anger at the four attacks, which killed more than 50 people.

"Curse this terrorism, it killed my friend," said retired businessman Ismail Yildirim. "The imperialist powers are behind this, they are turning the Middle East into a bloodbath. They want to drag Turkey into it but they will fail."

Pinned to his lapel was a photograph of a friend killed in last Saturday's suicide attack on the Neve Shalom synagogue.

Some in the crowd of several thousand held up Turkish flags and stood motionless in the bright winter sunshine for a minute of silent remembrance.

One group laid a wreath of red carnations at a monument in the square, below a statue of modern Turkey's founder, Ataturk.

Nearby a leftist group chanted anti-American slogans, shouting: "Terrorist America get out of the Middle East."

"We know who the murderers are," said a caption on one placard, below pictures of U.S. President George W. Bush and British Prime Minister Tony Blair.

Turkey is a close NATO ally of the United States, but many Turks believe U.S. Middle East policies played an indirect role in provoking the Istanbul attacks.

AGAINST VIOLENCE AND TERROR

Some in the crowd also suggested that U.S. and other intelligence services were directly responsible.

"I believe these attacks carry the fingerprints of the CIA, Mossad and (Turkish intelligence service) MIT. They want to push Turkey and America closer together after recent tensions between them," said 22-year-old engineering student Mustafa Colak.

He pointed to strains in ties between Ankara and Washington after Turkey refused to allow the deployment of U.S. troops on Turkish soil ahead of the Iraq war, and Ankara's recent decision to withdraw its offer of troops to help U.S. forces there.

Among those gathered in Istanbul's Taksim Square were also some who saw the bombs as an attempt to undermine Turkey itself.

"The groups behind this don't want to see a powerful Turkey in the Middle East," said metal union official Mehmet Soyupek.

"Things were going well for Turkey, the economy is recovering, and they want to put a stop to that."

Several hundred people turned out for a similar protest in Turkey's capital, Ankara, and about a thousand in the western coastal city of Izmir. "Against violence and terror - Peace," said one Ankara banner.

"We are against all terrorism, all violence, all over the world," said retired banker Muge Gulses. "The USA supports terrorism. England and the USA and developed countries sell guns to underdeveloped countries... They only think about money."

In the Istanbul crowd was at least one casualty of Thursday's attack on the HSBC Bank building -- Alper Tolga Akkus, 30, who had been near a window on the seventh floor.

He had a bandage above one eye and walked with a crutch but came to the protest to show the bombing would not cow him.

"The terrorists will not change my life," Akkus said. "They want us to be scared but I will not be intimidated." (Additional reporting by Gill Tudor in Ankara)