KHARTOUM, Sudan (AP) _ Legislators on Monday approved the new government's program, which includes a peace plan to end the 6-year-old civil war in southern Sudan.

The peace plan endorsed the suspension of Islamic law. Suspending Sharia is a key demand of the rebels, most of whom are African-descended Christians or animists. Northern Sudan and the government are dominated by Moslems, and fundamentalist Moslems have vowed stiff and even violent opposition to the removal of Islamic law.

It generally had been assumed that legislative approval of the peace plan would automatically freeze action on a bill to amend the existing Sharia-based penal code. But the bill is scheduled to go to the assembly Tuesday, indicating the issue has not been resolved. Provisions of the amended law, drawn up by a parliamentary committee, have not been disclosed.

After Monday's vote, assembly speaker Yusuf Ahmed Yusuf said, ''Endorsement does not necessarily mean dropping other bills.''

The remark by Yusuf, a member of the fundamentalist National Islamic Front, was seen as a challenge to the promised suspension of Islamic law.

Sudan's Constituent Assembly voted 128-23 for the policy statement Prime Minister Sadek Mahdi presented after creating a peace Cabinet under pressure from the armed forces. The colonels had demanded Mahdi make peace with the rebel Sudan People's Liberation Army or step down.

About 100 legislators abstained from voting on the measure, but Mahdi's Umma Party and its coalition partners command more than 200 seats to the Islamic Front's 50.

Mahdi's peace plan calls for a cease-fire, an end to a state of emergency in force since 1985 and a constitutional conference that would deal with a final settlement with the insurgents.

On March 26 Mahdi's government adopted the peace plan based on one negotiated in November with rebel leader John Garang.

Garang has said he would open peace talks with the government only after legislators approved the plan including the suspension of Islamic law.

The Islamic penal code provides for amputation of limbs for theft, execution by stoning for adultery, and public flogging for drinking alcohol.

The newspaper Al-Khartoum on Monday quoted Islamic Front Secretary-General Ali Haj Amin as saying his party ''will resist any move aimed at blocking ratification of the Islamic bill.''

Last week the front's regional secretary, Mohamed Osman Mahjoub, told a rally: ''We will mobilize all Moslem households and call all Sudanese Moslems to come out in the streets to fight for their pride.''

Islamic law has not been practiced in Sudan since President Gaafar Nimeiri was ousted by the military in 1985.

Mahdi tried to defuse the issue Sunday, telling reporters, ''We are making an effort to reach peace, and we have decided that (action on) Islamic laws be postponed until the convening of the projected constitutional conference.''

That indicated the proposed constitutional conference would have the final say on the issue.

Garang's rebels began fighting in 1983, demanding greater autonomy and a better economic deal for the southern zone. No reliable figures are available on how many people have been killed in the civil war.

Western relief agencies estimate that in 1988 the fighting and a war- created famine killed between a quarter million and a half million southern Sudanese.