President Robert Mugabe launched his re-election campaign Friday with a fiery warning to a 15-member southern African regional bloc that Zimbabwe could pull out amid demands for a free and fair vote.

"Let it be known that we are in SADC (the Southern African Development Community) voluntarily. If SADC decides to do stupid things, let it be known that we can withdraw from SADC," he told a 20,000-strong rally in the capital.

The regional bloc has pressed Mugabe to roll back his decision to hold elections on July 31, in order to allow time for a series of reforms that would limit the military's role in politics and strip ghost voters from the electoral roll.

Zimbabwe's constitutional court ruled on Thursday that the election would go ahead, despite Mugabe's request for a two-week delay.

His longtime rival and Prime Minister Morgan Tsvangirai had sought an even longer delay.

As he issued the threat to leave SADC, Mugabe also scolded Lindiwe Zulu, a envoy of South African President Jacob Zuma, who has been the chief mediator on the Zimbabwe crisis.

"Did such a person think that we as a country would take heed of this streetwoman's stupid utterances," said Mugabe, without mentioning Zulu by name.

The 89-year-old Mugabe issued a feisty rallying call to some 20,000 supporters, as he endeavours to extend his 33-year rule.

"You are our soldiers. You have a battle to fight. Go into the battle well-armed.

"Go into the battle with the full knowledge that there is a political enemy. This is a do or die struggle."

Mugabe's party lost its parliamentary majority to Prime Minister Morgan Tsvangirai's party at the last elections for the first time since independence in 1980,

Tsvangirai won the most ballots in the first round of the the 2008 vote, but pulled out of the second round amid violence against his supporters.

Some 200 opposition activists were killed around those polls

Amid the backdrop of violence the international community pressed Mugabe into a power sharing government with Tsvangirai.

The elections will choose a successor to their four-year old administration.

Mugabe urged supporters to avoid violence this time round.

"Let's kick our opponents with votes. But please no violence. Let's have an election without violence, without intimidation."

Under a new constitution Mugabe could serve another two five-year terms, or until he is 99 years old.

"We want to succeed. We need a political life," he said at a rally held at Zimbabwe Grounds, in the capital's Highfield township where he gave his first speech emerging from seven years of a guerrilla warfare against white minority rule in 1979.

But he and his party face an uphill struggle to win over voters, many of whom are disgruntled at the poor state of the economy that has forced millions of Zimbabweans to emigrate to neighbouring countries.

New elections were supposed to have been held 18 months after the formation of the power-sharing government in 2009 but were delayed by frequent disagreements among the parties over electoral reforms.

Observers have warned that elections without reforms will be doomed.