ABUJA, Nigeria (CNN) -- Nigeria's iron-fisted military ruler, Gen. Sani Abacha, died Monday after suffering a heart attack, state officials announced.

"General Sani Abacha passed away in the early hours of this morning ... May his soul rest in peace," Chief of Defense Staff Major-General Abdusalam Abubakar said in a statement.

He said Abacha would be buried at his home city of Kano on Monday in accordance with Muslim rites. There were unconfirmed reports saying Abacha had been buried already.

The statement did not give the cause of death but family members and Western diplomats in Lagos said it was a heart attack.

It was not immediately clear who would take charge of the West African nation of at least 104 million. Abacha's former deputy was sentenced to death in April for plotting to take power and never replaced.

Top Nigerian officials convened an emergency meeting Monday and the flag was lowered to half-staff at the Nigerian Mission to the United Nations in New York.

Political tension was already running high ahead of the August 1 presidential elections for which Abacha was the only candidate after his adoption by all five officially approved political parties in April.

At least seven people died in opposition protests against Abacha's rule last month and more demonstrations were planned before the elections despite the arrest of dozens of activists.

Abacha's transition to civilian rule, announced under pressure from at home and abroad in 1995, had been criticized as undemocratic by both local opponents and Western countries that imposed limited sanctions to press for reform.

Abacha took over at the helm of state in 1993 after canceling presidential elections and suspending the constitution.

The Associated Press contributed to this report.