Thailand's opposition is set on Tuesday to demand the annulment of last weekend's election and the dissolution of the ruling party, in a legal offensive that threatens to intensify the kingdom's political agony.

Prime Minister Yingluck Shinawatra called Sunday's snap election in a bid to defuse mass rallies that have dogged her government for three months, sparked deadly violence in Bangkok and thrust the kingdom into prolonged political turmoil.

The latest legal move by the opposition, which plans to file a legal challenge to the polls in the Constitutional Court later on Tuesday, came as the United States warned against any moves to stage a military coup, in the most outspoken remarks yet by a major ally.