The offensive by Al Qaeda in Iraq, a Sunni extremist group, left at least 100 people dead, in what the Iraqi authorities described as an ambitiously staged sequence of 40 attacks that covered a broad area of the country. The attacks reinforced fears that the civil conflict in Syria, which has become increasingly sectarian in nature, now threatened to spill over the border.

The attacks followed a declaration by Al Qaeda in Iraq's leader, Abu Bakir al-Baghdadi, drawing parallels between its hostility to the Shiite-led government in Iraq and the predominantly Sunni revolt against President Bashar al-Assad of Syria, whose Alawite sect is closely aligned to the Shiites.

Mr. Baghdadi, in a 33-minute speech posted Sunday on a Web site often used for messages by Al Qaeda, promised that a new offensive, which he called Breaking Down Walls, would begin soon. He described the impending campaign as part of a battle by Sunnis against Iraq's Shiite leaders and people.

He also heaped praise on the Syrian insurgents, declaring, "You have taught the world lessons in courage, jihad and patience," according to a translation by the SITE Institute, a Washington group that tracks Islamic extremism on the Internet. It quoted him as saying the anti-Assad fighters in Syria had rendered the Syrian president and his allies "terrorized by the future of your volcano."

The offensive, coming in the early days of Ramadan, the monthlong religious rite of fasting by day and feasting by night, was without precedent over the past few years, at least in the sheer number of attacks, spread over so many locations in a third of Iraq's 18 provinces, from north to south.

It raised new concerns about the government's ability to contain the violence, six months after the last American troops left the country following more than eight years of occupation and civil war that upended Saddam Hussein's Sunni-led minority power base and empowered Iraq's long-repressed Shiite majority.

"I think Al Qaeda in Iraq made a big joke of the government and the Iraqi security forces," said Khalid Fadel, a military analyst and former instructor at the Iraqi Military College. "They were so clear that they were going to launch attacks during Ramadan, and the government said that they have information of about 30 terrorist groups entering the country, but still the security forces are unable to prevent the attacks."

The first attack came at 5 a.m. on Monday when gunmen stormed onto an Iraqi military base near the town of Dhuluiya in Salahuddin Province and killed 15 Iraqi soldiers, according to security officials. Four soldiers, including a high-ranking officer, were wounded, and a fifth was taken prisoner by the insurgents.

Then, in steady succession, mostly from 6 a.m. to 10 a.m., car bombs exploded across the country, from Taji and Husseiniya north of Baghdad, to Sadr City in eastern Baghdad; in Tuz Khurmato in western Salahuddin Province; Dujail in southern Salahuddin; and Balad and Baquba, northeast of the capital, according to police, hospital and Iraqi Army officials. Bombs were also detonated in the northern cities of Mosul and Kirkuk, and in Diwaniya Province in the south.

The insurgents also attacked the home of a police official in Balad, seriously wounding four family members, and ambushed a checkpoint near Baquba, killing one policeman.

By early afternoon the violence had mostly subsided. Then from 5 p.m. to 7 p.m., Iraqi officials said three more car bombs exploded in Baghdad, killing three people and wounding 18. The dead included an Iraqi brigadier general in a blast on a convoy in central Baghdad.

Political analysts said Mr. Baghdadi's attempt to link his group's struggle to Syria was alarming but not surprising, given the evidence that Sunni extremists are now operating in Syria and presenting themselves as allies of the Sunni-led insurgency there.

Iraqi leaders have expressed concern before about what they have called movements by operatives of Al Qaeda in Iraq along the porous Iraq-Syria frontier.

Yasir Ghazi reported from Baghdad, and Rod Nordland from Cairo. Reporting was contributed by Duraid Adnan from Baghdad; Iraqi employees of The New York Times from Baghdad, Diyala, Kirkuk, Mosul and Samarra; and Rick Gladstone from New York.