UPDATE 2-Nigerian police and Shi'ite group clash in capital, one dead -Reuters correspondents

Reuters correspondents saw Nigerian police and Shi'ite Muslim protesters clash in the capital on Monday with at least one marcher appearing to be dead, highlighting one of the security challenges faced by the country with Africa's biggest economy.

A youth leader who was among the Islamic Movement of Nigeria (IMN) protesters said he saw six dead bodies. The police, who did not immediately respond to requests for comment on the report, tweeted that adequate measures were taken to quell what it said was a violent protest. The clashes in Abuja's business district, which lasted for about an hour, underscore the security problems in Africa's top oil producer and most populous nation - a country that is central to regional stability.

In recent days bandits in the northwest killed at least 37 people, Islamist insurgents are suspected to have kidnapped aid workers in the northeast and pirates abducted Turkish sailors in the Gulf of Guinea. IMN members regularly take to the streets of Abuja to call for the release of their leader, Ibrahim Zakzaky, who has been in detention since 2015 and they say requires medical help. Live ammunition and teargas have reportedly been used by security forces in recent weeks.

Reuters correspondents heard gunshots and saw bloodstains on a usually busy road in the capital. The National Emergency Management Agency (NEMA) said IMN protesters set two of its vehicles on fire. IMN youth leader Abdullahi Muhammed said he saw more than 20 casualties including people who had been shot on their legs and stomach. "I have seen six corpses now," he said, adding that police had many of the bodies.

He said the protest began as a peaceful march but police "started shooting with live ammunition on us".

"SECURITY FOR EVERY CITIZEN"

Nigerian broadcasting company Channels TV said in a tweet that one of its reporters was shot in the stomach. The police did not immediately respond to text messages seeking comment on the alleged casualties.

The inspector general of police said he had briefed President Muhammadu Buhari on the situation. "The president asked us to make sure we provide security for every citizen of this country and not to leave any space that some group of people will create breakdown of law and order," he told reporters in Abuja.

Clashes between police and Zakzaky's supporters have raised fears that IMN may become radicalised - just as the Sunni Muslim militant group Boko Haram turned violent in 2009 after police killed their leader. Zakzaky has been held in detention since December 2015, when the army killed roughly 350 of his followers at his compound and a nearby mosque and burial ground in northern Kaduna state.

He is facing trial for murder, culpable homicide, unlawful assembly, disruption of public peace and other charges following the 2015 violence. He was charged last year and has pleaded not guilty. Buhari, a former military ruler, began a second four-year term in May after winning an election in February following a campaign in which he vowed to improve security nationwide. He has repeated that pledge in the last few days.

The presidency, in a statement on Friday, urged IMN members to cease their protests on the grounds that the matter was being dealt with in the legal system and not the government.