Gen Prayuth said Thursday he did not mind the rally taking place but its organisers should adhere to democratic ideals and not speak of the army in an unfavourable way.
"I would like to ask whether this government came from democratic means and if everyone wants democracy? If so, we should follow the [democratic] rules," he said.
"Do not put pressure on officers and criticise me negatively. That would be disrespectful. If you condemn the army, you are condemning not only Gen Prayuth but also 200,000 soldiers. If you do that, do not expect anything from us," the army chief said.
Gen Boonlert Kaewprasit, Pitak Siam's leader and the rally's core organiser, earlier said Gen Prayuth has the power to stage a coup to protect the country and that a coup would happen one day.
He said that the "Stop the Crisis and Catastrophe" rally was intended to expose former prime minister Thaksin Shinawatra's influence over the "puppet government" led by his sister Yingluck Shinawatra.
Deputy Prime Minister Chalerm Yubamrung gave assurances yesterday that the rally will be peaceful.
Mr Chalerm spoke after having lunch with Gen Boonlert at the Royal Turf Club. Mr Chalerm later told the Bangkok Post that Gen Boonlert had not given the government any reason to believe the rally would get out of control.
Mr Chalerm added that no political issues were discussed over lunch.
He said Gen Boonlert had simply agreed not to let the rally drag on beyond 6pm.
Gen Boonlert said Mr Chalerm had not asked him to call off the rally.
Prime Minister Yingluck Shinawatra said yesterday the Pitak Siam group has the democratic right to hold its rally on Sunday as long as it remains peaceful and stays within the boundaries of the law.
Meanwhile, former prime minister and Democrat Party leader Abhisit Vejjajiva said his party does not support a coup.