Nigerian troops have been deployed in the northern city of Kaduna to help police restore order after riots flared over the introduction of Islamic Sharia law.

At least 25 people have been killed in the unrest between Christians and Muslims, in what is the most serious threat to the nation's unity since the end of military rule last May.

Kaduna's acting state governor, Stephen Shekari, told the BBC the situation was being brought under control.

He has declared a curfew from midday to 0600, with warnings that violators will be "shot on sight".

About 1,000 police and army personnel are enforcing the curfew.

A BBC correspondent in Kaduna says only sporadic shooting can be heard in the city centre, and the situation is far calmer than the previous evening.

Burned alive

Large parts of Kaduna are devastated, with hundreds of vehicles and houses having been set on fire.

In the southern suburbs, Christian youths have been roaming the abandoned streets, carrying clubs and axes and cans of petrol chanting: "No to Sharia law".

Hundreds of civilians have sought refuge in army and police barracks - with Christians hiding from Muslims and Muslims hiding from Christians.

Police said troops from the One Mechanised Division based in Kaduna had been sent to help restore calm as the state government ordered businesses and schools to close.

A BBC correspondent in Kaduna said he had seen a number of people killed, some of them burned alive. Witnesses reported dozens of bodies on the streets.

People said police fired indiscriminately at rioters who were refusing to retreat. Several buildings - including churches and at least one mosque - were seen burning

The violence flared after thousands of Christians took to the streets on Monday protesting against the proposed introduction of Islamic law in Kaduna State.

Eyewitnesses said counter-chants by Muslims who support Sharia quickly degenerated into fighting.

Obasanjo anguish

Nigeria's President, Olusegun Obasanjo, expressed anguish at the situation and urged Christians and Muslims to settle their differences peacefully.

More than 1,000 people have died in unrest since he became president in May last year.

Despite the fears of Christians, the Kaduna authorities have made no firm commitments to establish Sharia law in Kaduna, but have set up a committee to study how it might be implemented.

The acting governor told the BBC he did not understand why people were so agitated.

The Sharia issue has become increasingly divisive in Nigeria since the announcement last October by another state in the north, Zamfara, that it would introduce the code.

Sharia courts began operating in Zamfara last month.

Although several rural states in northern Nigeria which are overwhelmingly Muslim have already announced their intention to introduce Sharia, Kaduna is a far more sensitive and politically important state with a substantial indigenous Christian population.

Muslims have repeatedly stressed that Sharia will not affect Christians.

They say that the Nigerian press, mainly controlled by Christians from the south of Nigeria, has exaggerated the issue.