Iraq's top Shi'ite cleric Ayatollah Ali Sistani complains that U.S. coalition forces have not secured Iraq's borders well enough to stop the infiltration of extremists. But he appeals for calm and unity in the aftermath of the bloodshed.

The top U.S. administrator in Iraq, Paul Bremer, has issued a statement condemning the attacks on Tuesday. And he warns that extremists will not succeed in stopping what he describes as Iraq's march toward democracy.

Shi'ite Governing Council member Abdul Aziz Hakim, who leads the Supreme Council for Islamic Revolution in Iraq, also has condemned the attacks.

He blames al-Qaida terrorists and Saddam Hussein loyalists for the bombings and other recent attacks he says are trying to provoke civil war in Iraq. He urged calm and patience.

"We demanded and demand the Iraqi people to tolerate and to be patient and try to restrain themselves in order not to allow the objective of these evils to be achieved, these evils who are the enemies of the Iraqi people, in order to avoid any sectarian problems," he said. "However, it is important to follow these suspected people and to recognize them and take the right legal measures against them."

No group has claimed responsibility for the attacks.

Iraqi politicians and U.S. officials are pointing to al-Qaida terrorists. Last month, U.S. official say they intercepted a message from one operative urging attacks to stir up civil war between Shi'ite and Sunni political forces.

Within hours of the attacks, council member Adnan Pachachi, a Sunni Muslim, also called for unity.

"And, it is important that our Iraqi people should be calm, should be patient and should continue their national unity and to make sure that the enemies of Iraq will not have the opportunity to inflict harm on our people," he said.

A top U.S. military commander says Iraqi police have apprehended one suspect in Baghdad and six others in Karbala.

U.S. Brigadier General Mark Kimmit says the explosions were caused by a combination of suicide bombs, remote-controlled explosives and mortar fire. He says attacks were well coordinated.

"This was not a pick up team, not an organization that just started," he said. "It clearly shows signs of a well coordinated organization with some level of sophistication."

The multiple explosions near Shi'ite shrines in Baghdad and Karbala, some 80 kilometers to the south, ripped through crowds of pilgrims from Iraq, Iran and other countries as they gathered to observe Ashura, one of the most solemn days of the Shi'ite Muslim year.