Separately, a suspected al-Qaida fighter in Iraq, believed responsible for the assassination of a US-allied Sunni sheik was arrested north of the capital, the military said on Sunday.
The late morning attack in Tuz Khormato, 130 miles north of Baghdad, came a day after the al-Qaida in Iraq announced a new offensive in Ramadan, the Islamic holy month that began last week.
Witnesses said a boy left the bicycle bomb near the cafe, which was located in a popular market and was one of the few open during daylight hours despite Ramadan.
Two of those killed were in the cafe, while three were in the market, police chief Capt Abbas Mohammed said. He added that 19 people were wounded.
Nobody claimed responsibility for the attack. But the Sunni insurgent umbrella group, the Islamic State of Iraq, said on Saturday that it was launching a Ramadan offensive in honor of Abu Musab al-Zarqawi, the founder of al-Qaida in Iraq who was killed by a US airstrike in June 2006.
It was the deadliest violence in a day that included a car bomb that killed one in western Baghdad, a mortar shell in eastern Baghdad that also killed one, and the shooting death of a police officer south of the capital.
Attackers blew up a school in Qarah Tappa, 70 miles north of Baqouba, days before final exams were to be held.
The bloodshed was a blow to government hopes that a peaceful Ramadan would demonstrate the success of the seven-month operation in the capital.
The government, meanwhile, faced a deepening political crisis with Saturday's announcement that anti-US cleric Muqtada al-Sadr's followers were withdrawing from the Shiite alliance in parliament. Al-Sadr's followers hold 30 of the 275 parliament seats.
The announcement, made to reporters in Najaf, means the Shiite-led government can count on the support of only 108 parliament members— 30 short of a majority.
However, it could probably win the backing of the 30 independent Shiite parliamentarians, as well as some minor parties.
Al-Sadr's decision will also sharpen the power struggle among armed Shiite groups in the south, which includes major Shiite religious shrines and much of the country's vast oil resources. 
till, the decision by al-Sadr's followers will complicate further US-backed efforts to win parliamentary approval of power-sharing legislation, including the oil bill and an easing of curbs that prevent former Saddam Hussein supporters from holding government jobs.
Al-Sadr's decision will also sharpen the power struggle among armed Shiite groups in the south, which includes major Shiite religious shrines and much of the country's vast oil resources.
The Sadrists had threatened to bolt the Shiite alliance for several days. But tensions rose after arrest warrants were issued against Sadrist officials in the holy city of Karbala in connection with last month's Shiite factional fighting there.
The warrants, which were made public Saturday, angered the Sadrists, who said the government was provoking them despite recent gestures by al-Sadr, including a six-month halt to military operations by his Mahdi Army militia.
Shiites have shown signs of increasing frustration with militia violence, much of it blamed on breakaway Mahdi Army factions and criminal gangs and extortion rings.
Internal clashes
Internal Shiite clashes broke out at a market in the Hamzah al-Gharbi area near Hillah, about 60 miles south of Baghdad, when shop owners from the Albu Jassim tribe fought back against militia fighters, leaving one civilian dead, a provincial police official said. The bullet-riddled bodies of a traffic police chief and his 11-year-old son also were found after they were kidnapped during the fighting, the official said, speaking on condition of anonymity because he was not authorized to release the information.
American commanders in southern Iraq have said Shiite sheiks are showing interest in joining forces with the US military against extremists, in much the same way that Sunni clansmen in the western part of the country have worked with American forces against al-Qaida in Iraq.
One of those clansmen, Sheik Abdul-Sattar Abu Risha, was assassinated outside his compound in the Anbar capital of Ramadi, just days after he met with President Bush.
The US military said one of those responsible for his death — Fallah Khalifa Hiyas Fayyas al-Jumayli, an Iraqi also known as Abu Khamis — was seized on Saturday.
"We do not assess that he was operating alone, there is an investigation and continuing operations that are focused on ensuring that all people who were involved in this attack or in this murder will be detained," said Rear Adm Mark Fox, a US military spokesman.
Iraqis in the predominantly Shiite area of Shaab in eastern Baghdad also rallied to demand that the government intervene to stop U.S.-led raids in the area. Demonstrators burned the American flag and changed anti-US slogans, but no violence was reported.