The European Union sent 130 election observers from 22 different countries across Cambodia Tuesday.
The EU observers will watch the opening of polling stations, the transmission of ballots and the vote count, said Elson Graham, deputy chief of the observer mission.
Graham said the main objectives of the election observers are to strengthen respect for human rights and the rule of law, to deter fraud , to enhance public confidence in the election process and to contribute to conflict resolution.
The EU will also follow complaints from political parties.
Sunday's election will utilize more than 15,000 polling stations. The National Election Committee has accepted 24,000 national observers and 400 international observers.
Hang Puthea, executive director of the Neutral and Impartial Committee for Free and Fair Elections in Cambodia, said the number of observers was not enough.
Koul Panha, executive director of the Committee for Free and Fair Elections, said he would send his 10,000 observers to the provinces on Friday.
NEC Secretary-General Tep Nitha said the observers were playing an important role to ensure the election would be free and fair.