Mugabe, Zimbabwe’s sole ruler for nearly three decades, is holding onto power despite economic and political turmoil that have forced him into a unity government with the opposition.
"There is no going back on the land reforms. Farms will not be returned back to former (white) farmers. That work will continue, but those farms have to be used properly.
"Again I want to say, the farmers who owned these farms, which now have been designated and offered to new owners must respect that law. They must vacate those farms, they must vacate those farms, they must vacate those farms."
Thousands of ZANU-PF supporters in party regalia turned up for Mugabe’s 85th birthday rally at a sports field at Chinhoyi University about 100 kilometres west of Harare.
Prime Minister Morgan Tsvangirai was not at the venue, despite earlier indications he would attend the rally.
Mugabe’s spokesman George Charamba said Tsvangirai had opted out of the event after realising it was organised by Mugabe’s ZANU-PF party.
"People should not read this as a snub. He (Tsvangirai) excused himself," Charamba told Reuters.
Mugabe told the crowd the Southern African Development Community (SADC) tribunal, which last year ruled in favour of a group of white farmers whose farms had been targeted for seizure, had no right to rule on the land seizures.
"Some farmers went to the SADC tribunal in Namibia, but that’s nonsense, absolute nonsense, no one will follow that," he told supporters at a rally to celebrate his birthday.
"We have courts here in this country, that can determine the rights of people. Our land issues are not subject to the SADC tribunal."
SADC finance ministers on Friday agreed to push for donor help to save Zimbabwe from economic collapse, which critics blame on Mugabe’s policies.
Zimbabwe’s new unity government is heavily reliant on donors to revive the country, which is suffering hyperinflation, food shortages and 90 percent unemployment.
Mugabe said the unity government will 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 percent, in certain companies we would have designated.
"In the areas of mining, agriculture and manufacturing, a methodological and systematic identification of areas in which the state and indigenous entrepreneurs can participate, is being carried out, in line with the Indigenous and Empowerment Act."
The nationalisation law, which Mugabe signed a few days before last March’s general election, seeks to transfer majority control of foreign firms, including mines and banks, to blacks.