The results were based on counts from 148 of 150 constituencies. The electoral commission said that final results would be released on Sunday.
The MMD said they expected to win the final two regions, but the PF announced they would be asking a court for a complete recount due to discrepancies between voter tallies and registration lists.
"I have evidence that results are being inflated ... They cheated me in 2006 and they want to do the same," Sata said in a conference hall in Lusaka, the capital, where officials were announcing the results.
Officials in Africa's largest copper producer have said that the army is on standby in case tensions escalate.
Ronnie Shikapwasha, the home affairs minister, said: "Defence forces are on alert in border areas and other places to quell firmly any violence."
Although Zambia is one of Africa's most politicaly stable nations, Sata made similar accusations after his election loss two years ago.
International and domestic observers said that the election was generally free and fair.
However, monitors said the poll was not without suspicion, particularly with regards to the marginal lead when about half of the 3.9 million population cast ballots.
Voting ended at 6pm (1600GMT) on Thursday and will determine the successor to Levy Mwanawasa, who died in August after suffering a stroke.
Mwanawasa combined anti-corruption, pro-foreign investment and stringent fiscal policy during his reign.