He said the party had "let the president down" in the first round and had not diverted enough energy into its campaign.
"In terms of strategy, we only applied 25 per cent of our energy into this campaign ... That [the run-off] is when we are going to unleash the other 75 per cent that we did not apply in the first case."
Later in the night, police cordoned off streets surrounding the York Lodge Hotel in Harare, the capital, where a New York Times correspondent, Barry Bearak, was among two foreign journalists arrested for operating without accreditation.
Wayne Bvudzijena, a national police spokesman, told AFP: "I can only confirm that we have arrested two foreign journalists at York Lodge."
|Tsvangirai has been Zimbabwe's most |
prominent opposition leader [GALLO/GETTY]
The opposition Movement for Democratic Change (MDC) has won the constituency-based parliamentary polls, but no results have yet been released for the presidential vote.
The MDC says Morgan Tsvangirai, its leader, has won based on its own tallies.
Matonga said: "We think, and it is my assumption ... there may not be a clear winner of the presidential one [vote] and it points to a re-run."
But Tendai Biti, the MDC secretary-general, said earlier that Tsvangirai had won 50.3 per cent against Mugabe's 43.8 per cent and urged Mugabe to concede defeat and avoid embarrassment.
Despite his party's proclamation of victory, Tsvangirai has refrained from declaring himself the president, a move seen as having helped prevent major unrest in Zimbabwe.
Police were manning a number of roadblocks for routine checks in the capital on Thursday, but there was no other sign of an overt security presence.
The Zanu-PF's politburo is to meet on Friday to discuss the elections, a party spokesman said.
"All I can confirm is there is a politburo meeting. That's enough, that's all I can say at the moment," Didymus Mutasa, Zanu-PF secretary for administration, said.
Earlier, Mugabe made his first public appearance since the March 29 elections, meeting Ahmad Tejan Kabbah, the former president of Sierra Leone, who is heading an African Union election observer team, at his residence in Harare, state television reported.
Asked about his meeting with Mugabe, Kabbah said: "He looked very relaxed, and is of the view that the problems of the country will be resolved amicably, and he is very relaxed about it."
Rumours of Mugabe's next move continue to circulate.
The South African financial daily, Business Day, reported that Mugabe had admitted to family and advisers that he had lost and was weighing up whether to concede or contest a run-off against Tsvangirai.
The newspaper said some members of Mugabe's government wanted him to see the contest through but personal advisers and his family want Mugabe to quit.
If a run-off vote is declared, it will be contested on April 19.