Pheu Thai Party de facto leader Sudarat Keyuraphan said on Wednesday morning the anti-junta bloc had mustered at least 255 MPs from seven parties to form a coalition government.
She said their shared aim was to stop the military junta from retaining power.
Negotiations were underway for more parties to join them, she said.
Represented at a press conference where the coalition was proposed were Pheu Thai, Future Forward, Seri Ruam Thai, Puea Chat, Prachchat and Phalang Puangchon Thai.
The New Economics Party was not represented but is expected to join the bloc.
Future Forward leader Thanathorn Joongruangruangkit said he supported Sudarat to become prime minister, given that Pheu Thai had won the most seats in Sunday’s election.
The bloc urged other parties to respect the people’s choice and join the anti-junta coalition.