BAGHDAD (AP) - Iraqi forces clashed with Shiite militiamen Tuesday in the southern oil port of Basra and gunmen patrolled several Baghdad neighborhoods as followers of Shiite cleric Muqtada al-Sadr ordered a nationwide civil disobedience campaign to demand an end to the crackdown on their movement.

Explosions rang out across central Baghdad as rockets or mortars fired from Shiite areas targeted the U.S.-protected Green Zone for the second time this week.

The violence was part of an escalation in the confrontation between the Shiite-run government and al-Sadr's followers - a move that threatens the security gains achieved by U.S. and Iraqi forces. At least 22 people were killed in the Basra fighting.

Al-Sadr's allies have grown increasingly angry over raids and detentions against them by U.S. and Iraqi forces, who insist the crackdown only affects rogue elements loyal to Iran.

(AP) Map locates Karbala, where a car bomb killed at least 39 people Monday Full Image

Al-Sadr's headquarters in Najaf also ordered field commanders with his Mahdi Army militia to go on maximum alert and prepare "to strike the occupiers" - a term used to describe U.S. forces - and their Iraqi allies, a militia officer said. He spoke on condition of anonymity because he wasn't supposed to release the information.

Al-Sadr has imposed a cease-fire on his militia fighters through mid-August, a move that is one of the key factors in a steep drop in violence over the past several months. But the truce is fraying.

Lawmakers from al-Sadr's movement announced in a Baghdad press conference that a civil disobedience campaign which began in selected neighborhoods of the capital was being expanded nationwide.

Stores and schools were closed in several other predominantly Shiite neighborhoods in the capital, apparently in compliance with the civil disobedience order. Armed Mahdi Army members were seen patrolling the streets in some Shiite neighborhoods of the capital.

In Basra, Iraqi soldiers and police battled Mahdi fighters for control of key neighborhoods in Iraq's second-largest city, 340 miles southeast of Baghdad. The fighting erupted a day after al-Maliki flew there to supervise a security crackdown against the militias.

(AP) Graphic shows major events on Tuesday in Iraq; 1c x 3 inches; 46.5 mm x 76.2 mm Full Image

Police and hospital officials reported that at least 22 people had been killed and 58 wounded in the clashes. Iraqi authorities on Monday imposed an indefinite nighttime curfew on the city.

AP Television News video showed smoke from explosions rising over the city and Iraqi soldiers exchanging gunfire with militia members.

Basra accounts for most of Iraq's oil exports, but an oil ministry official, declining to be identified because he wasn't supposed to publicly discuss the sensitive issue, said production and exports had not been affected by the fighting.

Curfews were also imposed in the Shiite cities of Kut and Nasiriyah.

In Baghdad, suspected Mahdi Army gunmen exchanged gunfire Tuesday with security guards of the rival Shiite Supreme Islamic Iraqi Council in Sadr City, police said.

The rising tension led many people in Shiite neighborhoods to stay at home rather than venture into contested streets.

Athra Ali, 27, a government employee who lives in the Hurriyah neighborhood, said she decided not to go to work after seeing many shops closed and streets abandoned.

A university lecturer at Baghdad's Mustansiriyah University also said the institution had closed early and sent students home.

In Basra, Col. Karim al-Zaidi, spokesman for the Iraq military, said security forces were encountering stiff resistance from Mahdi Army gunmen.

The U.S. military said Tuesday that five suspected militants were killed in Basra while attempting to place a roadside bomb. Ten others were injured after being spotted conducting suspicious activity, the statement said.

Security in Basra had been steadily declining well before the British handed over responsibility for security to the Iraqis on Dec. 16.

British troops remained at their base at the airport outside Basra and were not involved in the ground fighting Tuesday, according to the British Ministry of Defense. Air support was being provided, but a spokesman could not say if it was U.S. or British planes.

"We have a capacity to provide air and other specialist support if needed, but at this time British involvement is minimal," the spokesman said, declining to be identified in accordance with department policy.

Last month, a British journalist working for CBS and his Iraqi interpreter were kidnapped from a hotel. The Iraqi was released after al-Sadr's office negotiated a deal, but the Briton remains in custody.

"We are calling for calm, but this new security plan has the wrong timing," Harith al-Edhari, the director of al-Sadr's office in Basra, said Tuesday. "This plan is a government scheme to target the Sadrists as they did in Diwaniyah and Muthanna."

Al-Sadr's followers have accused the Shiite-dominated government of targeting their followers in advance of provincial elections expected this fall. They have demanded the release of supporters rounded up in recent weeks.

The cleric recently told his followers that although the truce remains in effect, they were free to defend themselves against attacks.

U.S. officials have insisted they are not going after Sadrists who respect the cease-fire but are targeting renegade elements, known as special groups, that the American military believes have ties to Iran. Tehran denies allegations it is fueling the violence.

In other violence, two bombs exploded in central Baghdad, killing one civilian and wounding eight others, and a U.S.-allied Sunni fighter also was killed in a drive-by shooting northeast of the capital, police said.

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Associated Press writers Bushra Juhi, Sinan Salaheddin and Qassim Abdul-Zahra contributed to this report.