Sata said on Monday that Mwanawasa had "stolen victory", but urged his supporters to remain calm after they staged violent protests on Sunday.
"Mwanawasa has over one million votes already. If the vote was not stolen I was going to beat him. But because the vote has been stolen I will not catch up," he said.
Sata said he expected that votes from remaining constituencies could only take his tally to a maximum of 900,000 votes.
With only 15 of 150 constituencies still to be declared, results certified by the Electoral Commission of Zambia gave Mwanawasa 1,065,732 votes or 43 per cent.
Sata trailed with 686,654 votes (27.7 per cent) while Hakainde Hichilema of the United Democratic Alliance was in third place with 668,396 votes (26.98 per cent).
Banks and businesses were closed in Zambia's capital on Monday after a night of looting by protesters who accused Mwanawas's party of rigging the vote.
For the second day, police fired tear gas to disperse protesters who had gathered in Lusaka's volatile Garden township, an opposition stronghold, witnesses said.
The main road from downtown Lusaka to the presidential palace was sealed off by police and heavily armed paramilitary units took up positions in front of banks and strategic government institutions, including the vote counting centre.
International observers have declared Thursday's voting free and fair but Sata said he has evidence that 400,000 ballots were spoiled or disappeared.
Mwanawasa now looks set to secure a second and final five-year term as president on the back of his economic record, which includes securing billions of dollars in debt relief and boosting economic growth.