PRAGUE, Czechoslovakia (AP) _ Authorities on Tuesday still were holding 15 of the 77 people detained during protests on the 20th anniversary of the Warsaw Pact invasion of Czechoslovakia, government spokesman Miroslav Pavel said.

A dissident source said those detained in Sunday's pdemonstrations included six leaders of a peace group that collected signatures on a petition demanding human rights and the withdrawal of Soviet troops from Czechoslovakia.

At least five signers of the Charter 77 human rights document were detained during the weekend. Dissident sources said at least four were released Monday, but Charter 77 spokesman Bohumir Janat was among those still detained.

The official news agency CTK said Monday that 77 people were detained Sunday when police charged demonstrators after a two-hour march by thousands of people chanting demands for freedom and democracy and the name of Alexander Dubcek, the leader of reforms crushed by the Soviet-led invasion in August 1968.

The demonstration was the biggest of its kind in 19 years.

Pavel said those held were gradually being released but those who were freed might face charges later.

Pavel accused the Western news media of creating a furor both about the demonstration and the anniversary of the invasion. He did not identify the media.

He said the West's disruptive aims were shown by a demonstration last Thursday by members of the Italian Radical party, who held a banner across Prague's Wenceslas Square demanding Soviet troop withdrawal, democracy and human rights.

At least 10 Radical Party members were detained and expelled after that incident, which Pavel called ''an attempt to create unrest and instability.''

He said 4,000 people ''at most'' took part in Sunday's protests. Western journalists estimated the demonstrations involved about 10,000 people.

The spokesman said the government of Premier and Politburo member Lubomir Strougal did not discussed Sunday's demonstrations at the Politburo meeting Monday.

Dubcek, leader of Czechoslovakia's communist government in 1968, had introduced reforms during a period known as ''Prague Spring.'' He was removed and his government replaced following the invasion.