The leading Roman Catholic official in East Timor has accused the Indonesian military administration of that former Portuguese colony of carrying out summary executions and mass arrests, according to a document released here in the Portuguese capital.

Carlos Ximenes Belo, apostolic administrator of Timor's capital, Dili, accused the Indonesian authorities of recruiting children into offensives against nationalist insurgents, killing peasants in reprisal for guerrilla attacks, and implementing forced migrations to community villages where conditons were "inhuman."

A Lisbon-based Catholic Church group monitoring the situation in East Timor said the four-page document, dated Jan. 1, 1985, was smuggled from the territory through "religious channels" to France and then Portugal.

Belo wrote a similar letter, dated Feb. 11, 1984, that was carried to Lisbon by the prelate who preceded him in the apostolic post. It ascribed ravages to the Indonesians allegedly resulting from stepped-up warfare against Marxist guerrillas. This year, he said "the war is clearly expanding."

The island of Timor, 400 miles northwest of Australia, formerly was divided between the Dutch, who transferred their western portion to Indonesia in 1949, and the Portuguese, who pulled out of the east 10 years ago.

With the Marxist Fretilin group ascendant there, Indonesia militarily annexed East Timor in 1976. Indonesia has imposed an effective news blackout by restricting access.

The church leader's report indicated Indonesian forces were reacting to widespread insurgency by the Revolutionary Front for Independent East Timor, or Fretilin, "with successive and systematic antiguerrilla sweeps."

This contrasts sharply with official Indonesian statements that the guerrillas have been reduced to a handful of demoralized men holding out in the inaccessible eastern mountains.

"The culture, ethnic identity and religious beliefs of the Timorese people are being threatened, violated and slowly destroyed," Belo wrote. "In the midst of the catastrophe that is devastating Timor, the church must speak out against the attacks on human dignity and condemn injustices."

Belo appealed "with the utmost urgency" for open, purposeful negotiations between all interested parties, in which he said "independence movements," apparently a reference to the Fretilin guerrillas, should take part "free from all coercion."

Census figures showed the population to be 650,000 in the early 1970s. Relief agencies estimate that famine, disease and the hostilities in the wake of the 1976 Indonesian invasion killed 150,000 to 250,000 Timorese. Indonesia denies the reports. The Indonesian-appointed governor, Mario Carrascalao, told reporters in February that Indonesia had won the support of the Timorese by improving living conditions.

Belo's document, which the church group said was originally addressed to the Indonesian authorities, contradicted this view. He said lack of sanitation had lead to widespread disease, and forced migrations prevented peasants from cultivating subsistence crops, leaving them dependent on infrequent supplies from the Indonesians.

Belo said the Indonesian armed forces carried out waves of arrests of suspected guerrilla sympathizers, including the "most simple and humble peasants." He accused the authorities of systematically replacing Timorese public servants with Indonesians and attempting to impose their own language and religion through control of the school system and intimidation of Catholics.