Mineral row
Both the MPRP and international observers say the polls were free and fair.
But Democratic Party leader Tsakhia Elbegdorj said his party was robbed of victory.
"If most people voted for us why did we lose? We lost because... corrupt people changed the results," he said.
This is the fifth election since Mongolia adopted wide-ranging economic and politic reforms in 1990.
Before that, its government was modelled on that of the neighbouring Soviet Union.
The MPRP ruled Mongolia from 1921 to 1996, when it was beaten by the Democrats. In 2004 the two parties were forced into an uneasy coalition but broke apart two years later.
The two parties disagree on how newly-found mineral reserves - copper, gold and coal - should best be exploited.