Attacks killed 52 in Iraq on Tuesday as some ministers began a boycott of government and officials delayed provincial polls, heightening tensions on the eve of the 10th anniversary of the US-led invasion.

At least 20 explosions and multiple shootings also left more than 170 people wounded in Iraq's bloodiest day in more than six months, reflecting the brutal unrest and endless political crises sparked by an invasion that had aimed to build a democratic ally in the heart of the Middle East.

The attacks come amid a spike in violence that has raised fresh questions about the capabilities of the Iraqi security forces, with separate reports by Britain-based Iraq Body Count and researchers in The Lancet putting the overall death toll from the decade of bloodshed at more than 112,000 civilians.

US President Barack Obama, meanwhile, marked the anniversary by paying tribute to the "sacrifice" of US troops, but had few words for the Iraqi people, promising instead in a muted statement to support wounded American veterans.

Most of Tuesday's attacks struck in Shiite neighbourhoods in Baghdad during morning rush hour, with security forces stepping up searches at checkpoints and closing off key roads.

Soldiers and police also established new checkpoints, and unusually, were scanning at least some government-marked vehicles that are typically allowed to pass uninspected.

In all, at least 15 car bombs were set off, including two by suicide attackers, along with multiple roadside bombs and gun attacks, officials said.

Tuesday was the deadliest day since September 9, when 76 people were killed.

It could have been much worse, however, as security forces claimed to have arrested 26 militants with dozens of bombs and missiles that they apparently planned to set off in Baghdad, state television reported.

No group immediately claimed responsibility for the violence, but Sunni militants often target Shiite civilians and government employees in a bid to destabilise Iraq.

Violence has spiked ahead of the 10th anniversary, with 116 people killed in the past week, according to an AFP tally based on reports from security and medical officials.

Powerful Shiite cleric Moqtada al-Sadr suspended his bloc's participation in sessions of Iraq's national unity cabinet later Tuesday, worsening an ongoing political crisis that has pitted Prime Minister Nuri al-Maliki against members of his government.

The move, which a Sadrist official said was in response to Maliki challenging parliamentary decisions in court, means five ministers loyal to the Shiite movement will not take part in weekly cabinet meetings until further notice.

The authorities also said provincial polls scheduled for April 20, Iraq's first vote in three years, were delayed in Anbar and Nineveh provinces, citing security concerns including threats to the lives of candidates.

Officials have not announced any ceremonies to mark the anniversary on Wednesday, with events more likely to be held on April 9, the day Baghdad fell.

Launched with the stated goal of wiping out Saddam's stores of weapons of mass destruction, which were never found, the focus of the divisive war quickly shifted to solidifying Iraq as a Western ally in an unstable region.

Though the war itself was relatively brief -- it began on March 20, 2003, Baghdad fell weeks later, and then-US president George W. Bush infamously declared the mission accomplished on May 1 -- its aftermath was violent and bloody.

While still high by international standards, the violence was only brought under some measure of control from 2008 onwards, as the American troop "surge" coincided with Sunni tribal militias deciding to side with US forces.

But political reconciliation, the strategic goal of the surge, was never fully achieved.

From territorial disputes in the north to questions over the apportioning of the country's vast energy revenues, a number of high-level problems remain unresolved, while Iraqis still grapple with daily struggles ranging from poor provision of basic services to high levels of unemployment.

Maliki's erstwhile government partners, meanwhile, have charged him with monopolising power, and little in the way of landmark legislation has been passed in recent years.