The believers met at their meeting point at a Buddhism centre in Nakhon Pathom at 9 a.m. before heading to the parliament. A Buddha image and what organisers claims is the world's biggest flag picturing the Wheel of Dharma led the procession.
Wiroj Poonsook, head of a Buddhist network, pledged the gathering would be peaceful.
Authorities have stepped up security at the parliament by bringing in police and elite border patrol police to guard the venue. Sniffer dogs have also been used.
Gen Thongchai Kuasakul of the Buddhism Promotion Foundation said he expected 200,000 Buddhist followers would participate in the rally.
The ruling military junta, known as the Council for National Security, yesterday agreed with a demand by Buddhist activists that Buddhism should be declared the national religion in the new charter.