Tens of thousands of South Koreans held a candlelit rally in the capital on Saturday to protest against the impeachment of President Roh Moo-hyun, hours after the acting leader appealed for calm.
Prime Minister Goh Kun urged military forces facing the communist North to be more vigilant after parliament sparked an unprecedented crisis by impeaching Roh on Friday.
In a demonstration of the unpopularity of that vote, police said some 50,000 people gathered in a protest rally in the heart of Seoul.
"This should not be happening to our country. I am so embarrassed and outraged," said 55-year-old housewife Cho Hyun-ok at the rally, sobbing.
Some in the crowd were members of Roh's Internet-based fan club Nosamo, or "Roh lovers' group". All wanted to show their dismay at Friday's political upheaval, when the opposition-controlled parliament sent the country into uncharted territory by voting to impeach Roh for breaking election laws.
A poll conducted by Yonhap News Agency on Friday showed nearly 75 percent of 1,018 South Koreans surveyed believed impeaching the president was wrong.
But the main opposition party, the Grand National Party, on Saturday again blamed Roh for the crisis racking a country trying to tackle a stand-off over North Korea's nuclear ambitions and to revive a fragile economy.
The party vowed to do its utmost to work with the acting president.
"Our armed forces are faithfully carrying out their duties by strengthening their security posture to forestall any gap in national security," Goh said in a televised address, adding that Seoul remained committed to six-way talks on North Korea's nuclear programmes.
The Defence Ministry reported no signs of unusual movements on the northern side of the most heavily fortified border on earth, where Pyongyang deploys many of its 1.2 million troops.
All Roh's powers are in limbo until the Constitutional Court rules on Friday's vote. That process could take up to six months, during which time the prime minister will run the country.
Defence Minister Cho Young-kil met the commander of US forces in Korea, General Leon LaPorte. The United States has 37,000 troops in South Korea, a legacy of the 1950-53 Korean War that ended in a truce and left the peninsula in a state of war.
"It is crucial that forces of South Korea and United States stay perfectly ready for crisis," the defence minister told reporters before the meeting.
The Defence Ministry said the "strengthened security posture" did not mean the forces were on a higher military alert status despite the political upheaval.
"The alert status is normal," said a ministry official.
Visiting US Homeland Security chief Tom Ridge said South Korea's political system was robust enough to cope with the shockwaves resulting from the impeachment.
The vote came barely a year into Roh's single five-year term. His time in office has been marked by dramatic and unsettling gestures such as a call for a referendum on his rule, statements of self-doubt and incessant media criticism.
Goh's address was the second in which he has tried to reassure the country and restore confidence in the economy since the 66-year-old veteran bureaucrat was thrust into the role of acting head of state on Friday.