South African whites are polarizing to new extremes on both the left and right of South African politics, according to the results of a municipal election yesterday in Johannesburg, the stronghold of liberal opposition.

In an election considered here to be a guide to South Africa's political future, the liberal Progressive-Reform Party won 19 seats, biggest share of the city council's 45 positions. It is a significant boost from the single seat the party won in the last regular election in 1972.

The biggest surprise was the gain of the Conservative National Party, South Africa's dominant political body, which gained five seats for a total of 16.

Meanwhile, the increasingly troubled official opposition group, the United Party, lost control of the council and dropped to 11 seats, prompting politicians and newspaper editorials to speculate that this could result in a new political realignment throughout the country.

The major factor in the new polarization, several politicians said, was the racial unrest last year that resulted in more than 500 deaths and vast destruction in African townships.

"It looks as if the voters were motivated by the belief that a stronger stand needed to be taken to prevent further violence," a member of the Progress-Reform Party said today.

"The split shows that whites are now looking either to the PRP to push changes that will alleviated the tension, or the Nats [National party] whose strong arm appears to have put down the disorders, at least temporarily," he said.

Perhaps most important to long-term developments, the election could eventually lead to a new realignment of parties, with the Progress-Reform party taking over from the United Party as the official opposition. The two parties spent several months discussing a merger but the effort collapsed earlier this year when the United Party split over the proposed joint platform.

Since then the United Party has fragmented, with increasing indications from several members that they plan to abandon the party for either the Progressives or Nationalists.

Connie Mulder, minister of information and interior and provincial leader of the National Party, put it succinctly today: "The United Party is disappearing."

Provincial leader of the United Party, Japie Basson, said: "Serious reflection demands that the opposition parties reconsider their position urgently."

The results also indicate that the National Party has not suffered from its tactics during six months of sputtering riots last year - strong police action and the arrest or detention of hundreds of suspected dissidents from all races.

However no party now holds a majority in the Johannesburg council. The bitterness with which the two major parties view each other over race policy could lead to at least a short term stalemate over actions in South Africa's most progressive city.