Morgan Tsvangirai, the new prime minister and former opposition leader, did not attend the celebrations despite earlier indications that he might.
Mr Mugabe's spokesman, George Charamba, told Reuters news agency Mr Tsvangirai had opted out of the event after realising it had been organised by the president's Zanu-PF party.
"People should not read this as a snub - he excused himself," Mr Charamba said.
Mr Mugabe turned 85 on 21 February but his party is being held a week later.
Mr Mugabe told the rally in Chinhoyi there would be "no going back" on planned and already executed seizures of land owned by white farmers.
Refugees in Johannesburg give their thoughts on Mugabe's birthday celebrations
The Southern African Development Community (SADC) tribunal in Namibia had no right to intervene on the farmers' behalf, he said.
"Farms will not be returned back to former farmers," he told the audience.
"Some farmers went to the SADC... but that's nonsense, absolute nonsense, no-one will follow that.
"We have courts here in this country, that can determine the rights of people. Our land issues are not subject to the SADC tribunal."
In November, the tribunal ruled that Zimbabwe's plans to seize dozens of white-owned farms were illegal under international rule and should be halted immediately.
Zimbabwe's Commercial Farmers Union says that, in the past month, operations at about 150 farms have been disrupted. Some farmers have been evicted.
Mr Mugabe said the new government would continue to push for a majority stake in companies operating in Zimbabwe.
"We would want to see a greater participation of our people in them, not less than 51%, in certain companies we would have designated," he said.
Mr Mugabe also urged his supporters to accept the new power-sharing government but his latest message on land reform is a sign that he is still firmly in control, BBC Southern Africa correspondent Peter Biles reports from Johannesburg.
This week, Mr Tsvangirai's Movement for Democratic Change said the government must intervene to stop farm disruptions, in order to enhance productivity.
The birthday celebrations come as Zimbabwe struggles with the world's highest inflation, food shortages and a cholera epidemic which the World Health Organisation says has killed 3,894 people since August last year.
The WHO reports more than 84,000 cholera cases in Zimbabwe
There have been more than 84,000 reported cases, says the WHO.
More than half the population is believed to need food aid, while just 10% of adults have a regular job.
Mr Tsvangirai - who was sworn in two weeks ago in a unity government with Mr Mugabe ending months of political deadlock - has said it will cost as much as $5bn to fix Zimbabwe's economy.
The country has asked for $2bn in emergency aid to revive public services and the business sector.
Following a two-day meeting of regional ministers in Cape Town, South Africa, members of the Southern African Development Community (SADC) and the African Union pledged to "pursue measures in support of Zimbabwe's economic recovery programme".
But Western donors have said they are waiting for proof that the unity government is really working before sending in funds.
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