BAGHDAD — A bomb exploded in the Iraqi parliament’s cafeteria in a stunning assault in the heart of the heavily fortified, U.S.-protected Green Zone Thursday, killing at least two lawmakers and wounding 10 other people.

The blast in the parliament building came hours after a suicide truck bomb blew up a major bridge in Baghdad, collapsing the steel structure and sending cars tumbling into the Tigris River, police and witnesses said. At least 10 people were killed.

The bomb in parliament went off in a cafeteria while several lawmakers were eating lunch, media reports said. In addition to the two dead, state television said at least 10 people were wounded.

After the blast, security guards sealed the building and no one, including lawmakers, was allowed to enter or leave.

A spokesman for the U.S. Embassy in Baghdad said no Americans were hurt in the blast.

The bombing came amid the two-month-old security crackdown in Baghdad, which has sought to restore stability in the capital so that the government of Iraq can take key political steps by June 30 or face a withdrawal of American support.

One of the dead lawmakers was Mohammed Awad, a member of the Sunni National Dialogue Front, said Saleh al-Mutlaq, the leader of the party, which holds 11 seats in Iraq’s legislature. A female Sunni lawmaker from the same list was wounded, he said.

A security official at the building said a second lawmaker, a Shiite member, also was killed. The official spoke on condition of anonymity because he was not authorized to speak to the media.

Mukhlis al-Zamili of the Shiite Fadhila party said the second dead lawmaker was a Kurd, adding that six of those wounded were members of the radical Shiite cleric Muqtada al-Sadr’s bloc.

Al-Zamili said he believed a suicide bomber wearing an explosive vest was behind the attack.

Another member of the National Dialogue Front, Mohammed al-Dayni, also suggested a suicide bomber was behind it.

“I am standing now at the site of the explosion and looking at the severed legs of the person who carried out the operation. If this tells us anything, it tells us that security is lax,” al-Dayni told Iraq’s Sharqiya television.

Earlier in the day, security officials used dogs to check people entering the building in a rare precaution — apparently concerned that an attack might take place.

The brazen bombing was the clearest evidence yet that militants can penetrate even the most secure locations. Masses of U.S. and Iraqi soldiers are on the streets in the ninth week of a security crackdown in the capital and security measures inside the Green Zone have been significantly hardened.

The U.S. military reported April 1 that two suicide vests were found in the heavily fortified region that also houses the U.S. Embassy and offices of the Iraqi government. A militant rocket attack last month killed two Americans, a soldier and a contractor. A few days earlier, a rocket landed within 100 yards of a building where U.N. Secretary-General Ban Ki-moon was holding a news conference. No one was hurt.

Khalaf al-Ilyan, one of the three leaders of the Iraqi Accordance Front, which holds 44 seats, said the attack was “aimed at everyone — all parties — our parliament in general being a symbol and a representative of all segments of Iraqi society.”

Al-Ilyan, who is in Jordan recovering from knee surgery, said the blast also “underlines the failure of the government’s security plan.”

“The plan is 100 percent a failure. It’s a complete flop. The explosion means that instability and lack of security has reached the Green Zone, which the government boasts is heavily fortified,” he said.

U.S. Embassy spokesman Lou Fintor said its officials were “investigating the nature and source of the explosion. No embassy employees or U.S. citizens were affected.”

Hadi al-Amiri, head of the parliament’s security and defense committee, said the explosion shook the building just after legislators ended their main meeting, and broke into smaller committees.

“A few brothers (fellow lawmakers) happened to be in the cafeteria at the time of the explosion,” al-Amiri told Al-Arabiya television. “But had they been able to place this bomb inside the meeting hall, it would have been a catastrophe.”

Al-Amiri added Iraqi forces are in charge of security in the building, and that explosives could have been smuggled in amid restaurant supplies.

A television camera and videotape belonging to a Western TV crew was confiscated by security guards moments after the attack.

Attacks in the Green Zone are rare.

The worst known attack inside the enclave occurred Oct. 14, 2004, when insurgents detonated explosives at a market and a popular cafe, killing six people. That was the first bombing in the sprawling region.

On Nov. 25, 2004, a mortar attack inside the zone killed four employees of a British security firm and wounded at least 12.

On Jan. 29, 2005, insurgents hit the U.S. Embassy compound with a rocket, killing two Americans — a civilian and a Navy sailor — on the eve of landmark elections. Four other Americans were wounded.

In addition to killing 10 people, Thursday’s bombing of the al-Sarafiya bridge wounded 26, hospital officials said, and police were trying to rescue as many as 20 people whose cars plummeted off the span.

Waves lapped against twisted girders as patrol boats searched for survivors and U.S. helicopters flew overhead. Scuba divers donned flippers and waded in from the riverbanks.

Farhan al-Sudani, a 34-year-old Shiite businessman who lives near the bridge, said the blast woke him at dawn.

“A huge explosion shook our house and I thought it would demolish our house. Me and my wife jumped immediately from our bed, grabbed our three kids and took them outside,” he said.

The al-Sarafiya bridge connected two northern Baghdad neighborhoods — Waziriyah, a mostly Sunni enclave, and Utafiyah, a Shiite area.

Police blamed the attack on a suicide truck bomber, but AP Television News video showed the bridge broken in two places — perhaps the result of two blasts.

Cement pilings that support the steel structure were left crumbling. At the base of one lay a charred vehicle engine, believed to be that of the truck bomb.

“We were astonished more when we saw the extent of damage,” said Ahmed Abdul-Karim, 45, who also lives near the bridge. “I was standing in my garden and I saw the smoke and flying debris.”

The al-Sarafiya bridge is believed to be at least 75 years old, built by the British in the early part of the 20th century.

“It is one of Baghdad’s monuments. This is really damaging for Iraq. We are losing a lot of our history every day,” Abdul-Karim said.

The al-Sarafiya bridge has a duplicate in Fallujah that was built later and made infamous in March 2004 when angry mobs hung the charred bodies of U.S. contractors from its girders.

“This bridge is linked to Baghdad’s modern history — it is one of our famous monuments,” said Haider Ghazala, a 52-year-old Iraqi architect.

“Attacking this bridge affects the morale of Iraqis and especially Baghdad residents who feel proud of this bridge. They (insurgents) want to demolish everything that connects the people with this land,” he said.

Before the al-Sarafiya bridge was destroyed, nine spans across the Tigris linked western and eastern Baghdad.

The river now serves as a de facto dividing line between the mostly Shiite east and the largely Sunni west of the city, a reality of more than a year of sectarian fighting that has forced Sunnis to flee neighborhoods where they were a minority and likewise for Shiites.

Baghdad’s neighborhoods had been very mixed before the war but hundreds of thousands of people have been displaced since then as militants from both Muslim sects have sought to cleanse their neighborhoods of rivals.

There have been unconfirmed reports for months that Sunni insurgents and al-Qaida in Iraq were planning a campaign to blow up the city’s bridges. U.S. military headquarters near the Baghdad airport and the Green Zone, site of the U.S. Embassy and Iraqi parliament and government, are both on the west side of the river.

Also Thursday, the U.S. military said its troops killed two suspected insurgents and captured 17 in raids across the country.