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HARARE, March 30 (Reuters) - Zimbabwe's opposition claimed victory on Sunday based on early results from an election in which it is trying to unseat President Robert Mugabe after 28 years of power.

"It's a historic moment for all of us. We have won this election, we have won this election," Tendai Biti, secretary general of the main opposition Movemement for Democratic Change (MDC), told reporters, diplomats and observers at a briefing.

The opposition has accused Mugabe of employing election-rigging tactics to stay in power despite Zimbabwe's economic woes and African observers say they detected fraud in Saturday's ballot.

Biti said his party's election agents had reported that early results posted at polling stations showed MDC leader Morgan Tsvangirai was projected to win 66 percent of the vote in the capital Harare, an opposition stronghold.

He said Tsvangirai had made significant inroads in Mugabe's rural strongholds by leading in the southern province of Masvingo and Mashonaland Central Province, north of Harare, where the MDC has not won a parliamentary seat since 2000.

Tsvangirai's winning trend had also extended to Mugabe's home province of Mashonaland West, where the MDC had taken a rural parliamentary seat, said Biti.

He said that in Zimbabwe's second city of Bulawayo, another opposition power base, Tsvangirai had a slight edge over former Finance Minister Simba Makoni -- also standing against Mugabe.

But Makoni was leading in Zimbabwe's southwestern Matabeleland South Province with 45 percent of the vote.

"This is just a sample of what we are getting from every province. In our view we cannot possibly see a (reverse) of this trend," said Biti, adding the MDC would announce more results at 0800 GMT.

The Zimbabwe Electoral Commission said it would start to announce official results later on Sunday.

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