Many of the red-shirt demonstrators were supporters of Mr Thaksin.
More than 40 parties fielded 3,832 candidates for the 500-seat lower house of parliament, the House of Representatives.
In a two-tier system of voting, 375 legislators will be elected by constituency, while 125 candidates will be chosen from lists according to the proportion of votes each party receives nationwide on a separate ballot. There are some 47 million eligible voters.
Ms Yingluck is a political novice and analysts say her popularity seems to rest on the fact she is campaigning on the policies of her brother, who many believe is Pheu Thai's real leader.