The network comprises residents living along the Mekong River in eight Thai provinces--Chiang Rai, Loei, Nong Khai, Bung Kan, Nakhon Phanom, Mukdahan, Amnat Charoen, and Ubon Ratchathani.
The group's secretary, Ittipon Kamsook, submitted letters to Senate Standing Committee representatives Sen Rosana Tositrakul, chairman of the Corruption Investigation and Good Governance Promotion Committee, and Sen Surachai Liengboonlertchai, chairman of Natural Resources and Environment Committee.
Mr Ittipon said most studies indicated there was no proof any technology would help reduce the impact of the dam on lives along the Mekong River once the Xayaburi structure is in operation.
The group submitted a letter addressed to Prime Minister Yingluck Shinawatra asking her to reconsider and annul the project.
Sen Surachai said the Xayaburi project is related to the Thai people, as the contractor is Thai and the project is funded by Thai banks, while the Electricity Generating Authority of Thailand (EGAT) will directly purchase the power generated by the dam.
This matter should be processed in accordance with the Constitution via public hearings conducted by civil society and independent environmental organisations, Sen Surachai said, adding that his committee will invite the premier and related parties to clarify the subject, with a proposal to reconsider the dam's affects which may have to the wider ecosystem.
Sen Surachai cited the Thai Constitution Article 67 Section 2 which gives the right to the people of Thailand to voice their opinions on any project tending to impact their communities in terms of environment and natural resources.
Meanwhile, Sen Rosana said those to be affected by the dam construction can check the Administrative Court's order on human rights issue in order to use as part of documents to further file the case to the court.
The move came ahead of an environment ministers' meeting of four Mekong nations --Cambodia, Laos, Thailand and Vietnam--opening Thursday in the Cambodian city of Siem Riep to decide whether to approve the controversial dam.