About 50,000 soldiers, police and other security personnel were deployed in Bangkok as caravans of protesters travelled to the city.
Department stores as well as office buildings were closed for security reasons as about 10,000 protesters gathered in area, Piya Utayo, a police officer, said.
'Poor against elite'
The United Front for Democracy Against Dictatorship has portrayed the demonstrations as a struggle between Thailand's impoverished, mainly rural masses and a Bangkok-based elite impervious to their plight.
The protesters are supporters of Thaksin Shinawatra, the prime minister ousted in a bloodless coup in 2006, whose policies of cheap health care and low-interest village loans benefitted the poor.
However, the mass protests over the previous three weeks have angered many Bangkok residents and prompted a rival demonstration on Friday.
More than 1,000 people wearing pink shirts held their own rally, saying that the red shirts were being unreasonable.
Al Jazeera's Aela Callan, reporting from Bangkok, said this was the first time people had come out in big numbers opposing the red shirts.
"The tension is rising in Bangkok ... The confrontation is getting more direct.
"The fact that pink shirts, as they are calling themselves, are coming out on the streets seems to indicate that people are starting to get very fed up with these protests."
Tourism industry workers say that the protests have scared off many tourists and cost the city a sizeable ammount in lost revenue.
The red shirts are demanding immediate elections and threatening more protests in coming days, extending a mass street rally that began on March 14 when up to 150,000 red shirts converged on Bangkok's old quarter.