Dozens of people are reported to have been injured in the clashes
Five people have died in violent protests in the Mongolian capital Ulan Bator over alleged electoral fraud, says Justice Minister Tsend Munkhorgil.
More than 300 people were hurt, he said, including many police. Hundreds of people have been detained.
The president has declared a state of emergency and curfew, and parts of the city have been sealed off.
Opposition supporters question early results from Sunday's parliamentary poll, which the ruling party won.
Preliminary returns suggest the Mongolian People's Revolutionary Party (MPRP) has taken at least 45 seats in the 76-seat parliament, but the opposition Democrats allege fraud.
Several thousand people gathered on to the streets of the capital after the preliminary results emerged on Tuesday.
The ruling party headquarters were set alight and government offices were looted. Paintings were destroyed by a fire at the national art gallery, Mongolia's Montsame news agency said.
Protesters set fire to the headquarters of the ruling party
Police used tear gas, rubber bullets and water cannon to force stone-throwing protesters back.
Justice Minister Munkhorgil said five people had died, but gave no further details of those killed.
A Japanese citizen - thought to work for a news organisation - was among the injured, he said.
Late on Tuesday, President Nambaryn Enkhbayar announced a four-day state of emergency.
"Police will use necessary force to crack down on criminals who are looting private and government property," said Mr Munkhorgil.
The capital has been placed under a 2200 to 0800 curfew, and alcohol sales banned.
By Wednesday morning some roadblocks remained in place, an Associated Press reporter in Ulan Bator said, but shops were open and transport was running.
Lawmakers and officials were to meet in the capital later in the day for emergency talks on the situation, Montsame said.
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.