At least 13 U.S. soldiers were killed and officials said American civilian contractors and Iraqi military personnel were also among the dead and wounded.
Reuters reported that an American Army official said the attack was likely the work of a rocket or mortar attack, but "a suicide bomber has not been ruled out." The insurgents launched the assault around noon as hundreds of soldiers filled the tent to eat lunch.
"The force of the explosions knocked soldiers off their feet and out of their seats. A fireball enveloped the top of the tent, and shrapnel sprayed into the men," wrote embedded Richmond Times Dispatch reporter Jeremy Redmon who was in the dining hall when the attack occurred.
"Amid the screaming and thick smoke in the tent, soldiers turned their tables upside down, placed the wounded on them and gently carried them into the parking lot," Redmon reported.
Redmon said that insurgents have fired mortars at the same dining hall more than 30 times this year.
The dining hall is built of cement blast walls topped by a tent canopy and can serve around 1,000 soldiers at one meal. It sits atop a hill inside Camp Marez, a forward operating base that is the home of three U.S. Army battalions from the 25th Infantry Division. Three hundred to 1,000 soldiers usually serve in a battalion. The U.S. Army and Iraqi security forces share the base.
After the attack, Redmon reported that "puddles of bright red blood, lunch trays and overturned tables and chairs covered the floor."
U.S. military officials in Iraq and Washington said their work in Iraq will go forward.
"It is a sad day in Mosul, but as they always do, soldiers will come back from that," said Brig. Gen. Carter Ham, the U.S. commander in Mosul.
President Bush, who visited injured soldiers in a hospital outside Washington, D.C. Tuesday, told reporters the attack was particularly difficult coming during the holiday season.
"We send our heartfelt condolences to the loved ones who suffer today," Mr. Bush said. "Just want them to know that the mission is a vital mission for peace. The idea of a democracy taking hold in what was a place of tyranny and hatred and destruction is -- is such a hopeful moment in the history of the world."
News agencies reported that a radical Iraqi Sunni Muslim group called the Army of Ansar al-Sunna claimed responsibility for the attack on its Web site. The group called the attack "martyrdom operation."
The same group has taken responsibility for other attacks and for the kidnapping and beheading of 12 Nepalese workers.
Insurgents also attacked a police station in Mosul on Tuesday but were repelled by Iraqi forces.
Earlier in the day, hundreds of students had protested in the streets of Mosul demanding that U.S. troops stop entering homes and mosques in the city, the Associated Press reported.
The attack came on the same day that British Prime Minister Tony Blair made a surprise visit to Iraq. In a statement before the attack, Blair called the conflict in Iraq "a battle between democracy and terror."