Demonstrators from the People's Democratic Force to Overthrow Thaksinism (Pefot) protesters are heading up the front of the march, followed by People's Democratic Reform Committee (PDRC) supporters from the group's Victory Monument rally site, members of the State Enterprises Labour Relations Confederation (Serc), and PDRC protesters from the Lumpini, Lat Phrao, Pathumwan, Ratchaprasong and Asok rally stages.
Mr Suthep is walking among PDRC protesters from Pathumwan.
The march, which set off at 10am, has brought in-bound traffic on Sukhumvit Road to a halt, but demonstrators are receiving a warm reception from people en route.
Today's march will end at the PDRC's Asok rally site.
Tomorrow, protesters plan to march from Lat Phrao to Ratchada.
Mr Suthep announced on Wednesday night that PDRC protesters would completely shut down Bangkok on election day this Sunday, Feb 2.Mr Suthep said the PDRC would not prevent people from voting as they did on advance polling day. However, he called on PDRC supporters to rally on every street and park their cars on roads.