About 100 participants in a symposium in Tokyo have called for Cambodia to hold the general election set for July in a free and fair manner.
Concern about the election is rising due to the increasingly restrictive stance of Prime Minister Hun Sen's government, which has been in power for more than 30 years.
Last year, the government made the leading opposition Cambodia National Rescue Party disband.
It also forced an English-language newspaper, The Cambodia Daily, to stop publication last year by demanding the payment of huge taxes.
The paper's deputy publisher, Deborah Krisher Steele, gave a lecture at the symposium held on Thursday by human rights NGO members and others.
She said Cambodian authorities have been employing similar methods to force other media outlets to shut down. She said the authorities want to silence witnesses of people being robbed of their freedoms.
Some participants said the Japanese government should follow the European Union in suspending its support for Cambodia's parliamentary elections.
Michiya Kumaoka has been engaged in humanitarian assistance for Cambodia for many years.
Kumaoka said the country's situation has significantly deteriorated. He said he will call on the Japanese government and other entities to help bring about fair elections in Cambodia.