The villagers, who travelled 300 km from the west coast province of Prachuab Khirikhan, said they would attend every hearing to protest any plan to site a power plant in their province.
Their protests in 2002 had persuaded the government to cancel plans for two coal-powered plants in the province.
Jintana Gaewkao, the protest leader, said: "Whenever there is a hearing, we all will go.
"If you want to build them, go and build on your homeland," read one of the placards carried by protesters.
They wore green T-shirts carrying a picture of a leading environmentalist who was shot dead in 2004.
The 2007-2021 power development proposal will be submitted for approval after a few more rounds of public hearings, said an energy official.
The draft currently proposes three scenarios, with varying investment costs and types of fuel.
A 1,000-megawatt nuclear power plant was an option in the proposal.
As part of the expansion plan for electricity production, Thailand needs to build 11 power plants with 700-megawatt capacity each.
Three of the plants are to be coal-powered and built in Prachuab Khirikhan.
In efforts to reduce dependence on natural gas, Thailand plans to use more coal, biofuels and nuclear power, and buy more electricity from Laos, Myanmar and China.
The expansion plan is based on the assumption that economic growth will average 5 per cent a year.
Bidding is expected to open later this year for contracts to build new power plants.