Large trucks were parked across both inbound and outbound lanes at tambon Wang Manao in Pak Tho district of Ratchaburi province.
The road is the main route to southern Thailand.
The farmers are from Ratchaburi, Kanchanaburi, Prachuap Khiri Khan, Phetchaburi, Nakhon Pathom and Samut Songkram.
Farmers' representatives are demanding that the government pay the amount owed for rice pledged under the rice pledging programme.
One of the government's showpiece policies, rice pledging has turned into a monumental failure. Losses from the scheme have run into hundreds of billions of baht while millions of tonnes of unsold, overpriced rice are deteriorating in warehouses across the country.
The current caretaker government does not have the authority to raise new funds and has run out of money to pay the farmers. Both state and private banks are refusing to lend to it because of the risks involved.
The farmers have vowed not to disperse until they get a firm answer from the government on the date of payment. They also said they would not vote on Sunday.
Police instructed motorists to use an alternate route involving an eight-kilometre detour.