Why has South Korea's president suddenly declared martial law?

After referring to the opposition's political attacks, President Yoon on Tuesday night said he was declaring martial law to "crush anti-state forces that have been wreaking havoc".

That put the military in charge temporarily - triggering the deployment of troops and police to parliament where helicopters were seen landing on the National Assembly's roof.

The military also issued a statement declaring all gatherings by parliament and political organisation would be banned, while media publications would also fall under military control.

But South Korea's political opposition immediately called Yoon's declaration illegal and unconstitutional. The leader of Yoon's own party, the conservative People's Power Party, also called his act "the wrong move".

Meanwhile, main opposition leader Lee Jae-myung called on his Democratic Party MPs to converge on the parliament to vote down the declaration.

He also called on ordinary South Koreans to show up at parliament in protest.

"Tanks, armoured personnel carriers and soldiers with guns and knives will rule the country... My fellow citizens, please come to the National Assembly.”

Hundreds heeded the call, rushing to gather outside the heavily- guarded parliament. Crowds of protesters chanted: "No martial law! No martial law."

Local media broadcasting from the site showed some scuffles between protesters and police at the gates. But despite the heavy military presence, the tensions did not escalate into violence.

And lawmakers were also able to make their way around the barricades to get into the parliamentary voting chambers.

Shortly after 01:00 Wednesday local time, South Korea's parliament, with 190 of its 300 members present, voted down the measure. President Yoon's declaration of martial law was ruled invalid.