The march will be the first activity that demonstrators at Urupong intersection and Lumpini Park will do together after their leaders decided last night to move ahead together.
"We will go to Silom together. It will be our first move together," Green Politics Group co-ordinator Suriyasai Katasila said.
He spoke after several thousand demonstrators marched from Lumpini to join the anti-government rally at Urupong Intersection, which also had thousands of protesters and has been going for a couple of weeks.
The protest at Lumpini was organised under the banners of the Dhamma Army and the People's Army Against the Thaksin Regime.
Chamlong Srimuang, who leads the Dhamma Army, marched from Lumpini alongside thousands of supporters to the Urupong rally.
Chamlong and Suriyasai were former leaders of the now-defunct People's Alliance for Democracy (PAD), which rallied hard against the Thaksin Shinawatra-led government in 2006.
The rally at Urupong, closer to Government House, has been led by the Network of Students and People for Thailand's Reform.
It remains unclear whether the demonstrators at Urupong intend to join a rally by the Business Club for Democracy. The club's leader Somkiat Homla-or said the club had asked its supporters to gather on Silom Road at 11.30am on Monday to get accurate information on the highly controversial bill.
"At 12.34pm, we will then blow whistles together for about one minute to make it clear that we are against the issuing of this amnesty," he said.