A prominent Islamic cleric has been shot dead inside a mosque in the northern Nigerian city of Kano.
Ustaz Ja'afar Adam and one of his followers were killed at dawn prayers.
He was once a key member of the Kano State government but has fallen out with the governor. The killing could be political, or a dispute between sects.
The death comes as President Olusegun Obasanjo warned that the government will "deal firmly" with acts of fraud and violence in forthcoming elections.
In a national radio and TV broadcast, Mr Obasanjo said "highly placed individuals" were encouraging violence.
This is seen as a thinly veiled attack on Vice-President Atiku Abubakar, who has fallen out with the president.
Haruna Idris, one of Mr Adam's disciples, told the AFP news agency that gunmen had shot the cleric twice.
"The two assailants rushed out of the mosque and jumped into a car with a driver at the wheel and sped off," he said.
Thousands of people had gathered at the mosque, AFP reports.
Mr Adam has been buried.
Police spokesman Haz Iwendi told the BBC News website that the killing "would not endanger tomorrow's elections in Kano."
"The shooting did not come completely as a surprise because Kano has always been a flashpoint of violence," he said.
Mr Adam is a member of the strict Saudi-backed Wahabbi strain of Islam and has also been involved in disputes with rival Islamic sects.
Earlier this week, two policemen were killed and 40 AK-47 assault rifles were stolen from a vandalised police station in the Sharada area of Kano.
BBC correspondents in Kano say already security has been beefed up in the north-western city with armed soldiers and riot police patrolling the streets.
In his national address, Mr Obasanjo said:
"It is a matter of great regret... that some supposedly highly placed individuals have been reported to be fanning the embers of discord, hatred, violence and destruction.
Police are on the alert to guard against campaign violence
"Let me assure all Nigerians that the federal government will leave no stone unturned in vigorously checking [any] sanctioning [of] acts of violence and thuggery and other excesses calculated to disturb the peace or disrupt the elections."
Mr Abubakar is barred from running for president because of corruption charges, which he says are political.
He has gone to court, seeking to be allowed to run but the case has not been decided, with just eight days to go until presidential elections.
Mr Abubakar says Mr Obasanjo declared two days of public holiday on Thursday and Friday in order to further delay his court case.
Mr Obasanjo says the holidays were to enable people to travel to where they registered to vote.
More than 100 people have died in election violence across Nigeria since last November, according to a recent report by Human Rights Watch.
Correspondents say tension is high in Ibadan, capital of south-western Oyo State, after killings at a political rally earlier in the week and extra soldiers have been deployed to the streets of the city.
In nearby Ondo State, a bomb exploded at a campaign office on Thursday. No-one was injured, police told AFP news agency.
Mr Obasanjo, who was re-elected in 2003, is standing down next month after eight years in power.
State legislative and governorship polls will be held on Saturday while the presidential and national assembly elections are due on April 21.
With Mr Abubakar's candidacy in doubt, the main contenders for president are the ruling party's Umaru Yar'Adua and former military ruler Muhammadu Buhari of the All Nigeria People's Party (ANPP).