CAPE TOWN, South Africa--National Party leader F.W. de Klerk was elected yesterday to a five-year term as president and was criticized immediately by conservative opponents for allowing an anti-apartheid protest that drew an estimated 20,000 people.

Police allowed two smaller protests to go on yesterday, even giving flowers to leaders of one march. Antiapartheid leaders announced plans for more activity today.

The Electoral College, dominated by Parliament members of the National Party, cast a unanimous vote for de Klerk, just one day after the march in Cape Town, the largest legal protest march in South Africa's history.

Moolman Mentz, spokesperson for the Conservative Party, the largest parliamentary opposition, said approval of the Cape Town march was "a knife thrust in the back" of the security forces. Mentz called for immediate Parliamentary debate.

In a rare move, the anti-apartheid Democratic Party voted with the Nationalists to defeat the Conservatives' motion.

In carrying out his stated policy to allow peaceful protest, de Klerk faces opposition not only from right-wing parties but from the security establishment that enjoyed wide powers under former President P.W. Botha.

As de Klerk addressed the Electoral College, anti-apartheid organizations announced that more demonstrations and marches were planned in Pretoria and Johannesburg today.

De Klerk, 53, has been acting president since August 15, a day after he and other Cabinet ministers pressured Botha to resign after 11 years in power. De Klerk will be inaugurated Wednesday in a Pretoria church.

De Klerk, whose party was elected by white voters, declared himself the leader of all South Africans, "not only those represented in Parliament."

"Our goal is that all South Africans, in a just and equitable manner, become part of the decision-making processes of South Africa," de Klerk said.

South Africa's 28 million Blacks, the majority, have no vote.

Emergency regulations imposed to quell racial unrest have allowed police to ban all outdoor rallies; anti-government meetings in churches, universities or private homes; speeches they consider subversive; and news coverage of protests and police action. They also permit police to detain anyone without charge.

Newspapers say there is division in the Cabinet on how to handle peaceful protests.

Foreign Minister Pik Botha and Finance Minister Barend du Plessis criticized police action in whipping Blacks off a white beach during the election campaign, while Minister of Law and Order Adriaan Vlok defended the action.

Vlok said in yesterday's Parliament debate that the organizers of unrest over the past six weeks "and no one else" were to blame for the deaths of innocent people during the election, but he acknowledged police action had resulted in injuries and deaths.

He said an investigation was being conducted into allegations of police misconduct in mixed-race and Black areas around Cape Town on election day.

Police were ordered to stay away from the center of Cape Town during the march Wednesday, which was held to protest police brutality.

The Star, the nation's largest daily newspaper, reported it had learned the decision was made over objections from security officials.

"Opinion in government quarters was that the go-ahead for the march was an important signal to the security establishment that the excessive influence in enjoyed during the P.W. Botha era was on the wane," the newspaper said.

The Rev. Allan Boesak, president of the World Alliance of Reformed Churches, appeared optimistic about de Klerk's election. "At last we have someone who is more pragmatic than ideological," he said.