The turnout is expected to go higher as the event continues into the night with about 5,000 police being deployed in the area near the Democrat headquarters.
Pol Gen Adul Saengsingkaew estimated the protesters could top 30,000, most of them from several areas in Bangkok. But he was confident that police numbers were enough to handle the rally. The police chief has ordered all police units to be on alert to coordinate with other government agencies to cope with any situation.
Army chief Gen Prayuth Chan-ocha called for a peaceful settlement to the political standoff and said the army would not get involved in the demonstration. He ruled out a military coup to solve the problem, as the political division was being deepened by the Pheu Thai Party bill, which had been revised to include convicted former prime minister Thaksin Shinawatra.
At the rally site, a stage and tents were put up in the afternoon. Centres for logistical support and food supplies were also set up, indicating the rally might be prolonged.
Police have closed Sethasiri road to traffic and a section of the local road from the Kamphaeng Phet intersection. Checkpoints have been set up to search people and vehicles entering the rally area for weapons and other illegal items.
Democrat members led by Suthep Thaugsuban, a Democrat MP for Surat Thani, attacked Prime Minister Yingluck Shinawatra and the Pheu Thai Party for intentionally amending the amnesty bill to help convicted former prime minister Thaksin. The blanket amnesty violated the rule of law, they added.
ML Apimongkol Sonakul told the protesters that the Democrats would fight to block the bill from passing through parliament.
Pol Gen Vasit Dejkunjorn, a co-founder of the Thai Spring movement, was on the stage with the opposition party to show his opposition to the "Thaksin regime'' and urged all Thais to come out to unite against it.
Another rally is being held at Uruphong with the same mission to oppose the Pheu Thai-led government.
Elsewhere, a group of red shirts led by Weng Tojirakarn and Natthawut Saikua gathered near the Thai Rath newspaper office on Vibhavadi-Rangsit road where taxi driver Nuamthong Priwal hung himself to protest the 2006 coup.
But the Village Chiefs and Kamnans Association of Thailand has thrown its support behind the government's blanket amnesty bill.
The organisation's chairman Yongyos Kaewkiew announced its stance in front of the parliament building on Thursday.
The amnesty bill, proposed by Pheu Thai MP for Samut Prakan Worachai Hemma, will help minimise political conflict and lead to national reconciliation, allowing the country to move forward, he said.
He argued that the legislation would benefit both politicians and members of the general public affected by political unrest in April and May 2010.
The most sensitive point of the amnesty bill is Section 3, which was changed during the Pheu Thai-dominated House committee to cover all involved in political unrest from the coup in 2006 to Aug 8 this year. The version passing the first reading before being sent to the panel was limited to protesters and the public.
The meeting was held as Prime Minister Yingluck led in a mobile cabinet meeting in Lop Buri and Sing Buri.
The opposition party attacked her for her absence and said she needed to take a position on the amnesty issue. Jurin Laksanavisit, the opposition chief whip, led the attack, saying the prime minister was trying to escape her responsibilities towards the bill, as it favoured her elder brother Thaksin, by using her cabinet schedule as an excuse.
Ms Yingluck has made little mention of the blanket amnesty.
House Speaker Somsak Kiatsuranont argued that the prime minister, who is a Pheu Thai list MP, was busy with mobile cabinet meetings.
Video by Sithikorn Wongwudthianun.
Video by Thanarak Khoonton.