The farmers were travelling on about 400 e-taen farm trucks, 10 ten-wheel trucks, 12 six-wheel trucks, 50 pick-up trucks and about 150 motorcycles.
The farmers spent last night at In Buri district of Sing Buri. When they set off this morning, their large convoy brought traffic on Highway 333 to a virtual halt.
Chada Thaiseth, a former Chartthaipattana Party MP for Uthai Thani who is leading the group, said the farmers would not return home until they had been paid.
He said some of the farmers would head to Suvarnabhumi airport, but did not explain why.
Farmers led by Rawee Rungruang, chairman of the Network of Thai Farmers, were waiting for the convoy to arrive at their Commerce Ministry rally site.
Mr Rawee insisted that the caretaker prime minister must meet the farmers in person to explain when they will be paid.