BAGHDAD, Iraq – The country’s most powerful Shiite cleric endorsed the draft constitution yesterday, rejecting opposition voiced by two popular leaders of Iraq’s majority sect and underlining a rift also on display in anti-British violence in the southern city of Basra.

Two officials in the Shiite Muslim hierarchy in Najaf said Grand Ayatollah Ali al-Sistani called senior aides together and told them to promote a yes vote among the faithful during the October 15 national referendum on the constitution.

The officials refused to be identified because they are not authorized to speak for Mr. al-Sistani, who issues statements only through his office and makes no public appearances.

Iraq’s minority Sunni Arabs, who lost power and privilege with the fall of Saddam Hussein in the American-led invasion, are deeply opposed to the constitution. They form the bulk of the country’s violent insurgency and have stepped up attacks on Shiites in advance of the vote.

Some saw a Shiite split in play during the violence this week in the predominantly Shiite city of Basra, where British troops clashed with mobs and smashed into a jail while rescuing two soldiers.

Anthony Cordesman of the Washington-based Center for Strategic and International Affairs, said the escalation of tension in Basra underscored the simmering rift among Shiite factions ahead of the referendum and parliamentary elections in December.

“In large part, this is a reaction to a struggle between hard-liners and more moderate religious elements,” he said.

Mr. Cordesman said the more moderate stance of the largest Shiite political party, the Supreme Council for the Islamic Revolution in Iraq, was not accepted in southern Iraq, where “a relatively hard-line religious takeover in Basra, one linked closer to Iran,” has created animosity toward the British presence.

Rioting broke out in Basra on Monday after British armored vehicles and troops encircled a jail where two British soldiers were taken after their arrest by Iraqi police. Rioters threw firebombs and stones at British forces, and TV cameras caught images of soldiers, some with their clothes on fire, jumping from burning vehicles and running from mobs. Five Iraqis reportedly died in the violence, but British soldiers suffered only minor injuries.

Later that night, British armored vehicles broke through exterior walls of the jail compound, smashed cars and demolished buildings in a rescue operation that freed the two soldiers who the British said were then in the hands of Shiite militiamen at a nearby house.

Basra authorities accused the British of violating Iraqi sovereignty, and the provincial governor ordered all Iraqis to stop cooperating with the British.

Yesterday, Governor Mohammed al-Waili said violators would face unspecified punishment. But later in the day, he said he was in negotiations with the British and the dispute was “about to be solved and the crisis ended.” He did not elaborate.

After the violence Monday, Britain pulled its forces off the streets, with patrols only returning to the city late yesterday and then only in armored vehicles.

“We need to understand that this is a warning,” Mr. Cordesman said. “Over the next few months we’re going to see a referendum on a constitution that’s going to trigger more debates on Shiite identity … The idea that the constitution or referendum is going to clear this up is unrealistic.”

Gunmen killed at least 11 Iraqis in five attacks yesterday, police said. Roadside bombers targeted American troops Wednesday night, killing one soldier and wounding six in Baghdad, the military reported.