Thousands of supporters of Thai Prime Minister Yingluck Shinawatra have joined a three-day rally on the outskirts of Bangkok aimed at countering months of sometimes violent anti-government protests.

In their largest show of force in months, the pro-government "red shirts", or United Front for Democracy Against Dictatorship as the group is formally known, said they were prepared to thwart any move to dismiss Yingluck, who faces mounting legal cases that could see her removed from office in coming weeks.