The government appears to be planning action to neutralize South Africa's main multiracial opposition political movement, the United Democratic Front, which formed just over a year ago.

After two months of harassment, during which 16 of the movement's 40 executive members have been imprisoned without trial under the security laws, Law and Order Minister Louis Le Grange gave an indication Friday of tougher action to come when he accused the front of pursuing the same revolutionary goals as the African National Congress, the banned guerrilla group fighting white rule in South Africa.

The United Democratic Front labels itself nonracial and its membership includes whites, Coloreds (the designation here for mixed race) and those of Indian origin, as well as the majority blacks. It takes as its charter the Freedom Declaration, an affirmation of human rights drafted by the African National Congress in 1955 and the only document of that body not to be banned.

Le Grange said the front was instigating a wave of unrest in black townships in which 80 blacks have been killed and more than 400 injured in clashes with the police over the past two months. He said it was doing this "to promote a climate of revolution."

Asked in a television interview two days later whether that meant he was going to outlaw the front, as the congress was outlawed 24 years ago, driving it underground, Le Grange was noncommittal, saying only that "we are monitoring the situation day to day."

If Le Grange did ban the front, it would represent a reversal of a U.S.-supported trend of lifting internal political sanctions. Since June last year, more than 60 banning orders on individuals have been lifted, leaving only 10 persons banned.

The remaining members of the front's executive held an emergency meeting in Johannesburg today to discuss the threat to their movement, which is a federation claiming as affiliates 645 labor unions and community organizations with a combined membership put at 2 million. Some members said afterwards they thought Le Grange might ban several individual leaders and some of the more important affiliate bodies rather than the front.

Among the affiliates are student, sport, and church groups, the Natal Indian Congress, the political movement founded by Mohandas Gandhi in 1894, and the white National Union of South African students.

"It seems there is definitely some kind of action pending," said Cassim Saloojee, an executive member.

"We challenge the government to ban the front," said Trevor Manuel, the acting general secretary, who is standing in for one of the detained officials. "It will discover that we represent the will of the people, and that cannot be banned."

The front does seem to have touched a popular chord since a young Colored preacher, Allan Boesak, floated the idea of forming it in January last year. The front was launched at an enthusiastic rally of 10,000 people seven months later.

Boesak's idea was to form a federation of organizations opposed to a new national constitution devised by the government of prime minister (now president) Pieter W. Botha, so that they could campaign more effectively against it.

The constitution offered a political voice to the people of mixed race and Asian origin but continued to exclude the black 73 percent from any national role.

The Botha administration hoped it would be seen as a significant reform of its apartheid system of racial segregation, and the Reagan administration welcomed it as such, but the front denounced it as tokenism. Boesak and others also saw it as a ploy to co-opt them and the Asians into a minority alliance with the whites against the Africans.

Although his idea took off with a rush, Boesak declined to accept the leadership of the front. He was elected president of the World Alliance of Reformed Churches last year and said he would use that as his main platform to oppose apartheid.

A broad-based executive drawn from the affiliate bodies was chosen to run the organization under three figurehead copresidents. This was partly to protect against its being crippled by a banning order against one or two vital leaders.

Boesak was named a patron, and has continued to be the front's chief adviser and strategist.

When the front was launched in August last year, it claimed 100 affiliates, with a total membership of 750,000. Today it has nearly trebled in size, according to Manuel.

It argues that the campaign against the constitution was a stunning success. Eighty percent of the registered Asian voters and 70 percent of the Coloreds stayed away from the polls. It called on those communities' elected members to resign their seats and on the government to scrap the constitution.

Neither obliged, but for the Botha administration it was a severe setback. The government's hardened line began then.

At the same time, the front began carving more of a niche for itself than just an alliance formed for the limited objective of resisting the constitution. Suddenly it was looking like the most substantial nationalist movement since the African Congress, and its swelling support suggested that the black masses saw it as such.

Nineteen of its leaders had been detained on the eve of these elections, presumably in the vain hope that this would cripple the boycott at the last minute.

Six of these leaders won a court application declaring Le Grange's detention notices invalid because he failed to give reasons for locking them up. Before the law minister could issue new detention notices, the six sought refuge in the British Consulate in Durban. This enabled the front to attract world attention.

Angered that Britain should allow the men to stay in the consulate, South Africa retaliated by refusing to return four South African nationals to Britain to face trial there on arms smuggling charges. The widespread condemnation this incurred has the protest movement claiming another victory in its campaign.

The front attracted further attention when three of the fugitives walked out of the consulate Saturday, to be immediately arrested in a busy downtown street and driven away to prison without charges or trial. The other three remain in the consulate.