The World Khmer Conference participants also demanded the immediate release of Kem Sokha and other political prisoners and called for free and fair elections.
A ruling Cambodian People’s Party spokesman said Sam Rainsy’s appeal to voters was “in vain”, adding that turnout was expected at between 60 and 70 percent.
Prime Minister Hun Sen on Monday blessed the Cambodian people during a Khmer New Year ceremony, declaring his victory over the opposition party.
Long Sokunthy, 42, represents some 300 people who are locked in a long-running land dispute with a local tycoon.
Foreign Minister Taro Kono said that Tokyo would help Cambodia with free and fair elections, according to a spokesman for his office, Norio Maruyama, who confirmed that Japan would supply ballot boxes for the July polls.
Cambodia National Rescue Party (CNRP) was dissolved by Supreme Court last November at request of PM Hun Sen's government, which alleged it was plotting to take power with the help of US.
Local nongovernmental groups also invited to monitor July vote ruling party is expected to win because main opposition party was dissolved.
The 85-year-old American academic was given the Award for Distinguished Contributions to Asian Studies for his extensive body of scholarly work on Cambodia’s history, politics and culture.
The Mekong River Commission is meeting in Siem Reap province this week to discuss alternative energy solutions, large-scale Chinese investment, and governance issues.
International delegates are required by law to be invited to observe Cambodian elections.
The claims come more than two years after the government passed a controversial law regarding the conduct of non-governmental organizations in Cambodia.
In March, a group of Cambodians in Europe gathered to protest at the United Nations Human Rights Council office in Geneva, Switzerland, as the body was due to review human rights situation in Cambodia.