YOBE, Nigeria (Reuters) - A suicide bomber killed at least 23 people in a procession of Shi'ite Muslims marking the ritual of Ashoura in northeast Nigeria's Yobe state on Monday, witnesses said.

The attacker joined the line of Shi'ites before setting off his device as they marched through a market in the town of Potiskum, in a territory at the heart of an insurgency by Sunni Muslim Boko Haram rebels, resident Yusuf Abdullahi told Reuters.

"I heard a very heavy explosion as if it happened in my room. It took place just 200 metres from my house," he said. Another person carrying an explosive that did not go off was also arrested, he added.

Mohammed Gana, whose brother was killed in the attack, said he counted 23 bodies at the scene.

Boko Haram's five-year-old campaign for an Islamic state, which has killed thousands, is seen as the main security threat to Africa's biggest economy and leading oil producer.

Ashoura marks the death in battle more than 1,300 years ago of the Prophet Mohammad's grandson Imam Hussein.