The intervention came after weeks of rising tensions between the military, which ruled the country for nearly five decades, and the civilian government over allegations of fraud in November's elections.

The military last week signalled it could seize power to settle its claims of irregularities in the polls, which Suu Kyi's National League for Democracy (NLD) party won easily.

Suu Kyi and President Win Myint were detained in the capital Naypyidaw before dawn on Monday, NLD spokesman Myo Nyunt told AFP, just hours before parliament was meant to resume for the first time since the elections.

"We heard they were taken by the military... With the situation we see happening now, we have to assume that the military is staging a coup," he said.

The military then declared, via its own television channel, a one-year state of emergency.

In Yangon, the former capital that remains Myanmar's commercial hub, troops seized the city hall, according to an AFP journalist.

Elsewhere, the chief minister of Karen state and several other regional ministers were also held, according to party sources, on the very day when the new parliament was to hold its first session.

The developments triggered a quick response from the United States and Australia, with both calling for the release of detained NLD leaders and the restoration of democracy.

"The United States opposes any attempt to alter the outcome of recent elections or impede Myanmar's democratic transition, and will take action against those responsible if these steps are not reversed," White House spokeswoman Jen Psaki said in a statement.

Australia said the military was "once again seeking to seize control" of the country.

"We call on the military to respect the rule of law, to resolve disputes through lawful mechanisms and to release immediately all civilian leaders and others who have been detained unlawfully," Australian Foreign Minister Marise Payne said.

- Communications restricted -

In the hours after the arrests, communications networks in Myanmar were restricted, with several mobile phone networks down.

NetBlocks, a non-governmental organisation that tracks internet shutdowns, reported severe disruptions to web connections.

Phone numbers in the capital Naypyidaw were also seemingly unreachable.

Myanmar's polls in November were only the second democratic elections the country had seen since it emerged from the 49-year grip of military rule in 2011.

The NLD swept the polls and was expecting to renew the 75-year-old Suu Kyi's lease on power with a new five-year term.

But the military has for weeks complained the polls were riddled with irregularities, and claimed to have uncovered over 10 million instances of voter fraud.

It has demanded the government-run election commission release voter lists for cross-checking -- which the commission has not done.

Last week, military chief General Min Aung Hlaing -- arguably the country's most powerful individual -- said the country's 2008 constitution could be "revoked" under certain circumstances.

Min Aung Hlaing's remarks, which came with rumours of a coup already rife, raised tensions further within the country and drew a warning from more than a dozen foreign embassies and the UN.

Myanmar has seen two previous coups since independence from Britain in 1948, one in 1962 and one in 1988.

Suu Kyi -- a former democracy icon and Nobel peace prize winner whose image internationally has been in tatters over her handling of the Muslim Rohingya crisis -- remains a deeply popular figure.

She spent 20 years off and on under house arrest for her role as an opposition leader, before she was released by the military in 2010.