The Reagan administration's point man on South Korea said yesterday that senior government and opposition figures in Seoul told him last week that widespread and continuous protests have altered the political situation there and that "changes are going to take place."

"I got the feeling almost as soon as I arrived that a sea change was taking place in South Korea," Assistant Secretary of State Gaston J. Sigur said of his mission to Seoul last Tuesday to Thursday.

He said the "basic message" from all points of the Korean political spectrum was "the situation is not as it was, and we all know it . . . . Changes are going to take place and have got to take place."

Senior State Department officials said that, despite this feeling of impending change, neither Sigur nor other Washington officials had warning that Chairman Roh Tae Woo of the ruling Democratic Justice Party would recommend on Monday, Seoul time, acceptance of direct presidential elections and almost all other objectives demanded by government opponents.

Testifying before the House Foreign Affairs subcommittee on Asia and the Pacific early yesterday afternoon, Sigur said that "very highly reliable sources" had told U.S. officials that Korean President Chun Doo Hwan would back his party chief's surprise proposals. Sigur testified about six hours before an announcement scheduled to be made by the Korean president.

The subcommittee hearing, arranged after the dramatic news from Seoul, was an unusual "love feast" session in which administration officials and members of Congress from both parties congratulated each other, as well as the South Korean people and political leadership, on what has occurred.

"It appears that South Korea, which has produced one of the great economic miracles of our time, will produce one of the great political miracles of our time," subcommittee Chairman Stephen J. Solarz (D-N.Y.) said.

He said credit should go, in order, to the Korean people, opposition political leaders, Roh and the Reagan administration and Congress for speaking with one voice in favor of reforms.

Rep. Jim Leach (Iowa), the subcommittee's ranking Republican, said the Reagan administration's backing for democratization in Korea was a praiseworthy turnaround from the administration's initial doubts about human rights, a key element in Carter administration foreign policy.

Delegate Ben G. Blaz (R), the nonvoting House representative of Guam, recalled the devastation in South Korea in 1953 when he served there as a U.S. Marine Corps lieutenant at the end of the Korean war. Democratic government in Seoul "took a few years beyond what I expected, but it's finally here," he said.

Sigur cautioned that many potential problems lie ahead in South Korea. "There is a long way to go," he said, even though he joined the lawmakers in celebrating the "good news" from Seoul.

In an opening statement, Sigur expressed hope that moves toward democracy in South Korea "will find repetition" in the one-party, communist North Korea.

Sigur said under questioning that "some military activity" was observed recently in North Korea but described this as not alarming. Another official said the change involved a higher priority given some military activity in the North than is usually the case at this time of year.