Maiduguri - At least 17 people were killed on Tuesday when an explosion ripped through a busy market in a mostly Muslim city in northeast Nigeria on a public holiday marking the birth of the Prophet Mohammed.
"From our preliminary reports, we have 17 dead and at least five injured from the blast in the post office area" in Maiduguri, the police chief for Borno state, Lawan Tanko, told AFP.
Tanko warned that toll could rise as casualties were taken from the bustling market to medical centres for treatment.
"These figures are likely to change by the time we get full reports from our men in the field," he said.
Initial reports suggested the blast was caused by a car bomb or a suicide bomber but there was no immediate confirmation from the authorities.
"The bomb was detonated in the midst of a large crowd of traders while a truck carrying firewood was passing by," said Tanko.
The explosion happened at about 1:30 pm (1230 GMT) in a part of the city hit previously by militants from the outlawed Islamist group Boko Haram.
"An explosive device concealed in a sack was abandoned near a butcher's stall by unknown persons around Kasuwar Jagwal," which hosts meat and grocery stalls, said trader Buba Adam Kolo.
Kolo said he saw "20 people lying lifeless" on the ground after the blast but added he could not tell whether they were dead or injured.
Another trader, who gave his name as Musa Abba, said: "I can't say if there are any deaths but it's possible because it happened during peak hours."
The explosion caused panic in the city, which like most of the rest of northern Nigeria is majority Muslim and was celebrating Eid Milad un Nabi.
"Everybody was running to save themselves," said Kolo, while other eyewitnesses described the scene as "pandemonium".
A police officer, who asked not be identified because he was not authorised to speak to the media, said the impact of the blast could be felt three kilometres (nearly two miles) away.
"We heard the explosion from the police station. The market has closed and traders are trekking back home in large numbers and they are passing by our police station.
"We are very much on alert."
City is spiritual home of Boko Haram
Maiduguri was the scene of a brazen Boko Haram raid on December 2 last year, when the insurgents burnt aircraft, seized weapons and razed buildings at military bases in the city.
Suspected Boko Haram gunmen killed five at a village market some 22 kilometres from Maiduguri on Sunday, burning cars, shops and tents storing grain in a deadly rampage.
Maiduguri is considered the spiritual home of Boko Haram, whose name roughly translates from Hausa as "Western education is sin".
The group, considered an international terrorist organisation by the United States, aims to impose a harsh form of Islamic law, sharia, across the country.
Thousands of people have died in deadly violence since 2009, both in militant attacks on schools and churches and as a result of the military response to the violence.
Attacks in urban centres such as Maiduguri, which is the capital of Borno state, have become rarer, as counter-insurgency operations have forced fighters out into remoter, rural areas.
Mobile phone networks have been largely switched off in Maiduguri since a state of emergency was imposed in May last year in a bid to prevent militants from planning attacks.