HARARE: Zimbabwean President Robert Mugabe on Monday buried his deputy, John Nkomo, whose death from cancer has underscored concerns about the 88-year-old leader's own health problems and succession plans.

Mugabe, who has been in power for more than three decades, is seeking another five-year term as president after his ZANU-PF party chose him as its candidate for elections due this year despite fears he is battling prostate cancer.

Making no reference to his own health, Mugabe called for peaceful elections and praised Nkomo, who died last week, as a man of principle who worked to foster political reconciliation between rival parties.

The southern African country has a history of violent and disputed elections, including one in 2008 that led Mugabe and arch rival Morgan Tsvangirai of the Movement for Democratic Change (MDC) to form a compromise unity government.

Mugabe told thousands of mourners at a Harare shrine to heroes of Zimbabwe's independence struggle that the best tribute to Nkomo would be a smooth election.

"Peace, unity and harmony should prevail in the country if we desire to move forward," he said in the hour-long address.

The Heroes' Acre speech was interrupted by heavy rain and a power cut to the public address system - a reminder of the economic woes that have bedevilled the former British colony since Mugabe's seizure of commercial farms in 2000.

Africa's oldest leader, who normally uses such platforms to attack his opponents, sounded conciliatory and urged Zimbabweans to bury "petty personal differences" over politics.

Despite the soothing words, critics say Mugabe is not doing enough to control militant ZANU-PF supporters, some of whom booed Tsvangirai when a cabinet minister acknowledged his presence before Mugabe's address.

Rights groups on Friday condemned what they called an escalating campaign against ZANU-PF critics ahead of elections, which could be held by September.

The civic organisations, including church and legal groups, said there was a "well-calculated and intensified" assault on rights activists, journalists and artists through slander, intimidation, raids, arrests and prosecutions.