In this story:

• Succession remains unclear
• An 'incredible opportunity'
• Rule marked by brutality
• Related stories and sites

ABUJA, Nigeria (CNN) -- Nigeria's iron-fisted military ruler, Gen. Sani Abacha, died Monday after suffering a heart attack, state officials announced. He was 54.

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

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

Radio Lagos reported that Abacha was buried at his home city of Kano on Monday in accordance with Muslim rites. Thousands of people lined the streets of Kano in northern Nigeria for the arrival of his remains.

African leaders, meeting at their annual summit in Burkina Faso, observed a minute of silence to pay their respects to Abacha.

Others were happy that Abacha was dead. "Thank God that evil man is gone," said opposition activist and lawyer Gani Fawehinmi. "Nigerian people will not allow the military to take over again."

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 was 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. 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.

The son of executed human rights activist Ken Saro-Wiwa said Abacha's death gives Nigeria a vital opportunity to restore democracy to the country.

"This is an opportunity for the opposition and an opportunity for democracy to be restored to Nigeria, though how that actually happens is anybody's guess," Ken Wiwa said in an interview with Britain's Channel Four television news.

Exiled Nigerian playwright Wole Soyinka, who won the 1986 Nobel Prize in literature, also said Abacha's death is an "incredible opportunity" for a return to democracy in Nigeria.

Soyinka said Moshood Abiola, a millionaire who won aborted elections in 1993 and was later arrested on treason charges by Abacha, should head a government of national unity.

Other nations also said the leader's death should be a catalyst for Nigeria to restore democracy.

Pope John Paul II visited Abacha in March

U.S. State Department spokesman James Rubin said the United States remains committed to promoting democracy in Nigeria and said he believes there should be a "genuine democratic process" that leads to the military transferring power to a civilian government.

The European Union called for an early return of democracy to Nigeria. A statement issued by British Foreign Secretary Robin Cook, whose country holds the rotating EU presidency, called for "the election of an accountable civilian government which will restore and respect human rights."

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 Nigeria in 1993 after canceling presidential elections and suspending the constitution.

His rule was marked by brutality. Critics faced certain arrest and sometimes execution if they challenged Abacha's military government.

Abacha rarely traveled, fearing coup attempts, and always wore dark glasses when he made one of his rare public appearances.

Planned elections in 1996 never materialized and Abacha was heavily condemned for hanging Saro-Wiwa and eight other political activists who were convicted by a closed military tribunal of murdering political opponents.

Saro-Wiwa often challenged the government and criticized the oil industry for environmental damage. Nigerian oil accounts for more than 80 percent of its international currency earnings.

After Saro-Wiwa's execution, the 53-member Commonwealth of Britain and former colonies took the unprecedented action of suspending Nigeria from the group.

Abacha was born September 20, 1943. He was a career soldier who trained in the United Kingdom and the United States in addition to Nigeria.

The Associated Press contributed to this report.