A rally site has been set up at the provincial sports arena in Phra Nakhon Si Ayutthaya district to accommodate supporters mainly from Samut Prakan, Nonthaburi, Pathum Thani, Sing Buri, Chai Nat, Ang Thong, Suphan Buri and Ratchaburi.
Nisit Sintuphrai led a group of key red-shirt figures on a boat trip around the island, where the old capital is located, to promote the event.
UDD members want to show their support for embattled caretaker Prime Minister Yingluck Shinawatra, who they see as under attack by the Constitution Court and other independent organisations. Mr Nisit accused them of a lack of neutrality.
The UDD and the Pheu Thai Party are at odds with independent institutions, including the National Anti-Corruption Commission, which has taken action against Ms Yingluck for dereliction of duty in the rice-pledging scheme. She chairs the National Rice Policy Committee, which supervises the money-losing programme.
The Constitution Court, meanwhile, this week declared the government's 2-trillion-baht infrastructure borrowing bill unconstitutional.
Surachet Chaikosol, an Ayutthaya red-shirt leader, promised that all key UDD leaders, including chairwoman Tida Tawornseth, her husband Weng Tojirakarn, Jatuporn Promphan and Nattawut Saikuar, would join the rally.
The gathering follows similar events in Udon Thani and Khon Kaen two weeks ago and a meeting held in Chiang Mai last weekend to rally support behind Ms Yingluck and press for the unsettled general election to be completed.
People's Democratic Reform Committee leader Suthep Thaugsuban on Friday night challenged the UDD to rally in Bangkok and pledged to call out anti-government demonstrators to show up to outnumber the red shirts if they came to the capital.
Pol Lt Gen Nares Nanthachot, chief of the Provincial Police Region 1, said more than 1,000 police were deployed in Ayutthaya for the UDD meeting. Officers were manning the tops of four high buildings to prevent snipers and grenade launches, he added.