They had begun setting up tents on the bridge, preparing to spend the night there.
A modified truck with sound equipment had arrived at the bridge, where the protesters were taking turns speaking, alternating with musical shows.
Gen Preecha said the Rama 8 bridge and the Wisutkasat intersection were being closed to protect the protesters from attacks.
Samdin Lertbut, a Pefot leader, said some protesters had been assigned to the Democracy Monument on Ratchadamnoen Avenue, to keep it as a stronghold after the People's Democratic Reform Committee (PDRC) moved away to join the shutdown at seven major locations.
On Monday afternoon, the main stage of Pefot will be relocated from Makkawan Rangsan bridge to the Phan Fa bridge, he said.