Six months of debilitating protests in Thailand have centered on whether to hold elections. The governing party dissolved parliament in December in an attempt to defuse the crisis and set the election for February. The opposition Democrat Party, which has not won a national election since 1992, refused to take part. Protesters called for an appointed prime minister and blockaded polling stations, leading to a court ruling that the election was unconstitutional.
The country's democracy was in deadlock.
Prayuth made the coup announcement Thursday flanked by senior military officers.
Supporters of the former government of Yingluck Shinawatra, a group known as red shirts, who were holding a demonstration Thursday on the outskirts of Bangkok, were dispersed by soldiers in black masks. The Thai media reported that their leaders were arrested.
The last coup in Thailand was in 2006 and was followed by more than a year of military rule.
Thousands of anti-government protesters remained on the streets at dusk but leaders warned the crowds that the military was on its way. "The military is coming but do not panic," said Samran Rodpetch, a protest leader.
In Thailand's turbulent political history, the king, Bhumibol Adulyadej, has on at least one occasion helped resolve political disputes. But he is 86 years old and ailing.
Prayuth said in his announcement that the king was "above the entire conflict."
The military "will protect and worship the monarchy," he said.
Prayuth referred to the people staging the coup as the "The national peacekeeping committee."
Anti-government demonstrators have sought to eradicate the country's most powerful political family, the Shinawatras. The country's politics have been turbulent since 2006, when the military removed Thaksin Shinawatra, the patriarch of the family, who founded a populist movement that has won every election since 2001.
Thaksin, whose power base was in the provinces, challenged the power of the Bangkok establishment, a clash that was at the root of the conflict. Thaksin lives overseas but remains very influential in Thai politics.