The move came a day after Democrat MP for Surat Thani Suthep Thaugsuban, who is leading the Samsen demonstration, said the party would wait for Senate deliberation of the amnesty bill before stepping up efforts to oppose it.
That announcement was strongly criticised by anti-government coalition members, including the yellow-shirt People's Alliance for Democracy (PAD).
Mr Suthep last night called on opponents of the blanket amnesty to gather at Samsen railway station at 10am tomorrow and "be ready" for the next move.
"If the crowd is large enough and the conditions are right, we will ramp up our protest," he told rally participants.
The Democrat Party said 50,000 people joined the rally on Saturday, but city police put the figure at less than 8,000.
"Those who disagree with absolving the guilt of political wrongdoers should come and join our protest tomorrow and listen to my announcement for the next move," he said.
The bill, passed by the Lower House last week, would grant a blanket amnesty to all people involved in political unrest, including protest leaders, soldiers and authorities responsible for ordering protest crackdowns, between 2004 and Aug 8 this year.
Critics say the move is intended to bring home convicted former prime minister Thaksin Shinawatra.
Mr Suthep urged the public to fly the Thai flag at their homes, offices and on their cars to demonstrate their disapproval of the blanket amnesty.
Democrat Party leader Abhisit Vejjajiva insisted that Mr Suthep would not betray supporters even if he was sentenced to death for murder charges relating to the 2010 red-shirt crackdown.
He was denying a rumour that Mr Suthep had made a deal with the government to compromise on the bill. "I have worked with him for 20 years. He may not listen to me sometimes, but he will definitely not betray you," he said.
Meanwhile, "silent" senators will prove the decisive factor in determining whether the contentious amnesty bill will be accepted for consideration by the Upper House, Senator Weerawit Khongsak said yesterday.
ACM Weerawit, a key member of the Group of 40 Senators, said the Senate is now divided into three groups. While government-allied senators support the legislation and senators led by the anti-Thaksin Group of 40 Senators oppose it, there are also senators who are still undecided whether to vote for or against the bill, he said.
The three groups each have equal numbers of members, he said.
On average, about 100 senators cast votes on any particular bill, while about 50 senators refrain from voting or do not attend, a parliamentary source said.
ACM Weerawit said if all senators were to vote on the amnesty bill, the extra 50 ballots could swing the fate of the legislation. He said when there is a secret ballot, this "silent" group of senators often votes against the government. If there is an open vote, they tend to abstain or are not present during votes.
It is up to Senate Speaker Nikom Wairachpanich to decide whether or not to hold a secret ballot. Mr Nikom has been criticised in the past for favouring the Pheu Thai government.
"When the Senate meets to vote on whether to accept the amnesty bill in the first reading, it will up to the undecided group of senators to decide whether the bill proceeds," ACM Weerawit said.
He said the Group of 40 Senators has resolved to not accept the bill in its first reading and will attempt to convince other senators to vote against it.
Political Development Council chairman Thirapat Serirangsan said he believed most elected senators with close ties to the government will rush to pass the amnesty bill without making any changes.
He said there are only three possible ways to stop the amnesty bill. The first is to petition the Constitution Court against the bill. The second would be for Democrats MPs to resign en masse to show that the legislative branch has failed and force a House dissolution.
The other option is for the anti-amnesty protesters to step up their demonstrations against the bill, Mr Thirapat said.