Prime Minister Hwang Kyo-ahn will be acting President for the duration of the court's deliberation.

He vowed to "run state affairs in a correct and transparent manner."

"I earnestly and humbly ask all of you to unite so that the voices on the streets can be sublimated into the driving force behind the effort to overcome the current national crisis," Hwang said.​

Under the South Korean Constitution, impeachment requires a two-thirds majority of the 300-member legislature to pass.

Thousands took to the streets to celebrate the news. National Assembly Speaker Chung Sye-kyun made the announcement, saying lawmakers had an obligation to restore order and to execute the functions of the government.

In a phone call with Defense Minister Han Min-koo, the acting President said that North Korea possibly could use the political upheaval to stir up trouble south of the 38th parallel and that the South Korean military should maintain its readiness.

"While retaining a watertight national defense posture, the government will work closely with the international community to thoroughly respond to the North Korean nuclear problem," Hwang said.

Two of Park's former aides also face criminal charges.

The impeachment is only the country's second. In 2004, late President Roh Moo-hyun was forced out of office for two months.

The Constitutional Court later restored Roh to power, rejecting charges of abuse of power and mismanagement.

What happens next?

The Constitutional Court's nine members need to return a two-thirds majority to oust Park.

In that case, a new election would be held within 60 days.

South Korean elections are "highly unpredictable. No one knows (who will win) until the last minute," said John Delury, associate professor of Chinese studies at Yonsei University's Graduate School of International Studies.

"It's going to be extremely difficult for a candidate from (Park's) party to win -- there is such deep ill will and discontent toward her and her party."

He added there are about half a dozen serious contenders for the presidency if Park is removed.

Much of the parties' platforms will focus on domestic issues, said Delury, with corruption and a separation between government and big business high on the agenda.

Why does this matter?

South Korea is a regional lynchpin and stalwart US ally, and the outcome of a general election, should the court rule that Park step down, could have security and economic implications in Asia and beyond. Should a liberal government come in the aftermath of Park's ouster, changing priorities could swing the balance in the region.

Delury said North Korean dictator Kim Jong Un has been "quiet on this whole thing. He's doing two things, letting Park stew and getting a lot of domestic points in North Korea for saying: 'Look how bad the South Korean government is.'

"That buys South Korea a bit of time as they get a new leader."

Liberal governments tend to engage more diplomatically with the North, as opposed to Park's policy of sanctions.

A new government may also choose to align more closely with China, which is strongly opposed to current plans to implement the American-made THAAD missile defense shield over the Korean Peninsula.

"If and when THAAD deploys, we can expect some serious economic and security countermeasures," Delury said.

"Depending on the timing, this could be the first problem a new South Korea leader faces."

There could also be significant economic implications, as some of the country's biggest conglomerates have suffered their own follies this year.

Dismal approval ratings

In a recent poll by Gallup Korea, Park's approval rating was at 5% -- up a point from her dismal previous showing of 4%.

It also indicated showed that 81% of those polled supported her impeachment.

"I will relegate the decision to the National Assembly, including the shortening of my presidential term and resignation," she said.

"If the National Assembly sets a path for the stable transition of power, I will resign from the presidency and lessen the confusion as much as possible. I hope that the nation will find stability."

But South Korea's President is immune from prosecution.