Iraq's security forces locked down key routes in Baghdad on Wednesday after the country suffered its deadliest violence in two months in the run-up to a landmark Arab summit due next week.

The measures came a day after a wave of nationwide gun and bomb attacks killed 50 people and left 255 others wounded on the anniversary of the start of the US-led invasion of Iraq.

The closures worsened already choking traffic in Baghdad, which has seen unprecedented levels of security as part of preparations for the first meeting of the 22-member Arab League to be held in the Iraqi capital in 20 years.

AFP journalists reported full or partial closures of key routes in the capital, while roads that remained open saw increased numbers of checkpoints and security forces, and virtually all of the bridges that traverse the Tigris were also shut.

The violence rocked 20 towns and cities spanning the northern oil hub of Kirkuk and the Shiite shrine city of Karbala, south of Baghdad, from 7:00 am (0400 GMT), and continued through the day.

The spate of attacks bore the hallmarks of Al-Qaeda, which typically tries to launch coordinated nationwide mass-casualty bombing campaigns, though no group immediately claimed responsibility.

The attacks were swiftly condemned internationally, with United Nations envoy Martin Kobler describing them as "atrocious", White House spokesman Jay Carney adding that the US "strongly condemns" the violence, and Britain's Middle East minister Alistair Burt slamming them as "cowardly."

Parliament speaker Osama al-Nujaifi condemned the "brutal criminal" attacks, and said they were part of efforts by Al-Qaeda to "derail the Arab summit, and keep Iraq feeling the effects of violence and destruction."

Following the attacks, the government declared a week of public holidays from March 25 to April 1.

Coupled with Kurdish New Year festival Nowruz on Wednesday and the weekly Muslim day of prayer on Friday, much of Iraq will be largely closed until after the summit.

Security forces have mooted the possibility of imposing a city-wide curfew on March 29, when Arab leaders are expected in Baghdad, the first such meeting to be held in the Iraqi capital since Saddam Hussein's 1990 invasion of Kuwait.

Officials insist Iraq's forces are capable of maintaining security for the summit, and have drafted in an additional 4,000-odd policemen and soldiers to do so, but admit they may need to effectively shut down Baghdad to do so.

Tuesday's violence was Iraq's deadliest day since January 14, when 53 people were killed in a suicide bombing outside the southern port of Basra.