Reuters could not open the websites of Radio Free Asia Khmer, Voice of America Khmer and Voice of Democracy on Friday.
The League for Democracy Party (LDP) is one of 19 parties registered to compete against the ruling Cambodian People’s Party and its leader, Prime Minister Hun Sen.
Human Rights Watch and the International Federation for Human Rights said in a statement that the election would pave the way for Hun Sen to cement his uncontested power in Cambodia.
Many Cambodians support the CPP due to its investment in numerous local development projects.
Congressman Ed Royce, chairman of the foreign affairs committee, said: “The people of Cambodia deserve far better than the Hun Sen’s despotism.”
The CPP has used many slogans during its campaign, including perhaps the most ubiquitous: “A CPP Win Means the Whole of Cambodia Wins”.
A VOA Khmer investigation in April found evidence of large-scale troop movements in late 2017 from the highly militarized provinces of Oddar Meanchey and Preah Vihear to Siem Reap province.
Amnesty is also calling for detailed legislation and a comprehensive code of conduct on the use of force by law enforcement personnel.
Under the election laws of Cambodia, preventing someone from voting can carry a fine of up to about $5,000.
The only major challenger to Cambodian People's Party was disbanded last year.
Japan sent election monitors to Cambodian elections in 1993, 1998, 2003 and 2008, but Chief Cabinet Secretary Yoshihide Suga said it would not be doing so this time.