South Korean public alarm over the projected withdrawal of U.S. troops surfaced in two demonstrations in Seoul today. In a third incident a leading opposition politician was injured while protesting President Park Chung Hee's authoritarian rule.

At midday, police dispersed an anti-withdrawal march through the captial by approximately 500 Christians, belonging to 19 Protestant denominations, who carried banners and placards declaring "absolute opposition to withdrawal for U.S. forces."

At a jampacked prayer meeting in a downtown church earlier, the withdrawal of U.S. ground forces was called an invitation to war. A three-point declaration said the GIs were the only method of deterring North Korean aggression and protecting 35 million democratic citizens.

All demonstrations are illegal in South Korea, and dozens of uniformed and plainclothes police blocked the hymn-singing demonstrators from access to the Seoul city plaza. They stripped the marchers of protest sashes and signs and herded them down into a subway station.

Another Christian group of about a hundred people assembled outside the residence of U.S. Ambassador Richard L. Sneider to protest the pullout. They were dispersed peacefully by police.

An opposition party convention ended in a melee with police when delegates of the Democratic Unification Part yemerged on the street chanting anti-government slogans. Police broke up the demonstration immmediately and at least 26 party supporters were detained temporarily.

The party has only three representatives in the South Korean National Assembly and a record of hostility to the Park government. A four-point resolution adopted today called for abolition of the present constitution, rescindment of presidential emergency decrees, release of political prisoners, and a guarantee of security on the Korean Peninsula through agreement of four major powers.

A spokesman, Hyur in Pai, added: "We want to restore democracy in Korea and the withdrawal of American forces should be reconsidered."

The party president, Yang II Dong, 65, fell during the scuffle with police, lost consciousness and was hospitalized.

Yang was to have attended a reception this evening given by President Carter's two special envoys. Under Secretary of State Philip Habib and Gen. George Brown, chairman of the Joint Chiefs of Staff. Habib and Brown completed a second day of discussions with Park and the South Korean ministers for defense and foreign affairs on the timing of the phase-out and compensatory measures to maintain the military balance in Korea.

Habib has met a number of opposition politicians, American missionaries and religious leaders in line with his promise to take a cross-section of public opinion.

Among the guests invited tonight are the Rev. Kim Kwan Suk, secretary general of the Korean National Council of Churches; former President Yun Po Sun, now under a five-year suspended prison sentence for opposing Park; and former Foreign Minister Chung II Hyung, who lost his National Assembly seat after being convicted in the same case as Yun.

An article in one of the government-controlled newspapers has attacked the invitations to convicted political dissidents as a challenge to South Korea's legal system.