(Removes erroneous word Sunni from paragraph 20)

* Deadliest day in Iraq this year

* Follows U.S. troop withdrawal from urban centres

* Iraqi security forces acknowledge negligence

(Adds U.S. State Department, paragraph 18) By Aseel Kami and Suadad al-Salhy

BAGHDAD, Aug 19 (Reuters) - A series of blasts in Baghdad killed 95 people and wounded 536 in Iraq's bloodiest day this year, prompting a rare admission of culpability from Iraqi security forces struggling to cope without U.S. help.

At least six blasts struck near government ministries and other targets at the heart of Iraq's Shi'ite-led administration, weeks after U.S. combat troops withdrew from urban centres in June, thrusting Iraq's security forces into the lead role.

"We must face the facts. We must admit our mistakes, just as we celebrate our victories," Defence Ministry spokesman Major General Mohammed al-Askari told a group of U.S. and Iraqi military officials on Wednesday through an interpreter.

His comments echoed earlier remarks by Baghdad's security spokesman, Major General Qassim al-Moussawi.

"This operation shows negligence, and is considered a security breach for which Iraqi forces must take most of the blame," Moussawi told Iraqiya state television.

The sectarian war that savaged Iraq after the 2003 U.S. invasion has abated and Iraq celebrated the restoration of its sovereignty in June when U.S. troops withdrew from cities.

The government this month ordered most blast walls in Baghdad to be removed within 40 days, a sign of faith in its troops and police before a national election in January. But Wednesday's coordinated attacks on heavily guarded targets shattered a growing sense of optimism about Iraq's stability.

In one blast, a massive truck bomb close to a checkpoint for the heavily fortified Green Zone blew out the windows of the foreign ministry. Dozens were mown down in a blizzard of glass.

"The windows of the foreign ministry shattered, slaughtering the people inside. I could see ministry workers, journalists and security guards among the dead," said a distraught ministry employee who gave her name as Asia.

SECURITY REVIEW

The explosion was powerful enough to blow in some windows of Iraq's parliament building in the Green Zone.

The attacks could undermine confidence in Prime Minister Nuri al-Maliki before the parliamentary election, and could also deter foreign investors, especially oil companies, which might have started to think of Iraq as a place to put their money.

Maliki called an emergency meeting to reevaluate the defence of Baghdad. He said in a statement that the attacks were aimed at "raising doubts about our armed forces, which have proven themselves very capable of confronting terrorists".

Analysts and members of the public disagreed.

"Today's attacks reveal a major deficiency and weakness of the security forces. They were organised and huge," said analyst Hameed Fadhel of Baghdad University.

Baghdad's normally busy streets emptied, and the few people still outside poured scorn on the security forces.

"The security forces don't provide security, they just cause traffic," labourer Haythem Adil said.

No group claimed responsibility, but Moussawi said two members of al Qaeda were arrested when another car bomb was intercepted. Iraqi television later showed a truck loaded with water tanks stuffed with explosives that had been disarmed.

U.S. State Department spokesman Ian Kelly in Washington said past coordinated incidents bore the "hallmark" of al Qaeda. "But I don't have any hard information by way of al Qaeda," he said.

Sunni Islamist groups like al Qaeda consider Shi'ites heretics, and have been blamed for a series of blasts in the last two months at mostly Shi'ite venues like mosques, both in the capital and in northern Iraq.

Iraqi politicians and other officials have accused neighbouring states, including Saudi Arabia, Iran and Syria, of fomenting violence in Iraq. Analysts say that could be a ploy to distract attention from domestic failings.

POWERFUL BLAST

The second truck bomb in Baghdad's Waziriya district close to the finance ministry killed at least 28 people and caused huge destruction. Part of a raised highway nearby collapsed.

"Suddenly a powerful blast shook the building and glass flew ... Most employees were wounded by the flying glass and others, including myself, suffered concussion ... I awoke with blood all over my face," said ministry worker Batoul al-Amri.

Another explosion was close enough to Reuters' offices in central Baghdad's Karrada district to burst open windows and doors. Columns of smoke could be seen rising from several sites.

The Baghdad provincial government building came under mortar attack, police said, as did the Salhiya district in central Baghdad, home to army bases and a television station.

At least one suspected mortar round landed near the United Nations compound in the Green Zone, startling U.N. workers marking the sixth anniversary of the destruction of their previous Baghdad headquarters by a truck bomb which killed envoy Sergio Vieira de Mello and other staff, U.N. guards said.

The U.S. military said it had no reports of mortar fire.

In Bayaa, in southern Baghdad, a blast killed two people. (Additional reporting by Ahmed Rasheed, Khalid al-Ansary, Michael Christie, Reuters Television and the Washington bureau; writing by Mohammed Abbas; editing by Tim Pearce)