The Organisation for Security and Co-operation in Europe (OSCE) has criticised Armenia's weekend election, saying it was tainted by instances of vote-buying and interference.

President Serzh Sarksyan's ruling Republican Party of Armenia won Sunday's elections, official results showed, laying the foundation for a new parliamentary system of government.

The OSCE said in a statement the elections were well-administered and fundamental freedoms were generally respected.

But, it added, they had been marked by organisational problems and undue interference in the process, mostly by party representatives. It also noted some pressure on civil servants as well as private sector employees.

"The elections were tainted by credible information about vote-buying, and pressure on civil servants and employees of private companies," OSCE said in the statement.

With 100 percent of the vote already counted, official results show the Republican Party won with 49.12 percent votes. The opposition Tsarukyan Alliance, led by wealthy businessman Gagik Tsarukyan, gathered 27.32 percent.

Central Election Commission chairman Tigran Mukuchyan said the figures were based on results from all 2,009 polling stations.

He said nothing about violations of election rules. The prosecutor's office said it had been reviewing media reports of violations during the voting and counting process.

Opposition parties said they had also filed complaints on irregularities to the prosecutor's office, but gave no further details.

The election was Armenia's first under a new constitution, approved in a December 2015 referendum. Parliament, rather than voters, will elect the president for the first time in 2018.

The new system reduces the presidency to a ceremonial post and increases the powers of the prime minister.