Nationalist guerrillas waging a sporadic war against Indonesian occupation forces on the remote island of East Timor have strengthened their military strike power under new leadership in preparation for a major counteroffensive, according to spokesmen in Lisbon.

The guerrillas, establishing their first contact with the outside world in four years, have given details of their regrouping in documents smuggled out to supporters abroad despite a rigorous information blackout imposed by Indonesian military authorities.

Abilio Araujo and Jose Ramos Horta, Central Committee members of the Revolutionary Front for an Independent Timor--known by its Portuguese abbreviation Fretilin--have shown foreign journalists in Lisbon what they say is most of the 60 pages of documents that are said to have reached them in December.

The spokesmen read transcripts that claim the resistance movement now has six separate units, totaling about 6,500 men. These forces, it is claimed, are spread through the west and center of East Timor in readiness to expand combat fronts against Indonesian forces, which are estimated to number 30,000. The guerrillas say they continue to dominate the mountainous eastern tip of the island, their traditional stronghold and refuge.

The claims contradict the official Indonesian position as well as reports from western diplomats in Jakarta that the resistance has been reduced to no more than 600 men, demoralized by lack of food and support and limited to one or two small raids a month.

Corroborating the resistance claims, however, are accounts of Timorese refugees recently arrived in Lisbon and letters from the island received here confirming increased guerrilla activity and insurgent successes.

Fretilin first came to prominence in November 1975 when it declared a short-lived democratic republic in East Timor, a former Portuguese colony 400 miles off the northwest tip of Australia. The movement had defeated two rival independence factions in a three-month civil war that broke out in the wake of the 1974 revolution in Portugal and the introduction of a decolonization policy.

The Portuguese armed forces and administration abandoned Timor at the outbreak of civil strife. Neighboring Indonesia, claiming Fretilin was a leftist threat to its security, immediately launched a full-scale military invasion from the western half of the island, which has been its territory since Indonesia came into being in 1949. Relief workers estimate that more than 100,000 islanders--one-sixth of the population--died in the famine and disease brought on by the hostilities.

Months later Moslem Indonesia proclaimed Roman Catholic-dominated East Timor its 27th province, a measure endorsed by successive U.S. administrations but repeatedly denounced by the United Nations. Fretilin withdrew into the mountains and began its guerrilla campaign for independence.

According to their smuggled communiques, the guerrillas have now reestablished a high command and secured communication channels to the outside, indicating a marked recovery from reverses in the late 1970s when many of their members were executed or imprisoned. Contact with supporters abroad was cut in 1979 when a prominent guerrilla abandoned Fretilin and handed their most efficient radio to the Indonesians.

Resistance leader Araujo, who fled during the Indonesian invasion to further the guerrillas' cause abroad, said: "For the first time we now have a top-rate command, coordinated inside and outside Timor."

He refused to indicate how the documents were smuggled out of Timor, which has been almost totally sealed off from outside scrutiny since the Indonesian annexation. But two points, at least, corroborate their authenticity.

A Timorese refugee newly arrived in Lisbon said there was "10 times more" guerrilla activity when he left last September than in 1980 and 1981. Others said fighting had been particularly intense during the state visit of Indonesian President Suharto to Washington in October, which the islanders had heard about on the overseas service of the British Broadcasting Corp.

Letters received by Timorese refugees in Lisbon who are not connected with Fretilin confirm many of the guerrillas' claims. A typical one, dated November 1981 reads: "The country has been cut off from every corner of the earth. We know little of what is being said about Timor. But what is certain is that the front (meaning Fretilin) has been considerably more active and is winning more territory."

Contacted in Amsterdam, Peter Hattinck, director of the Portuguese service of Radio Netherlands International, confirmed that a letter forwarded to him from the smuggled package indicated that a broadcast that mentioned Timor had been picked up on the island. "I believe the letter is genuine," he said, and he has acknowledged it in a subsequent program.

Most of the clandestine documents, which are dated Oct. 14, 1982, are signed by Sha Na Na, the Timorese name of the 38-year-old former colonial administration employe, called Jose Alexandre Gusmao in Portuguese, who is now widely held to be Fretilin's top commander.

According to the papers, the Fretilin leadership has been regrouped under the new name "The Revolutionary Council of National Resistance." The guerrillas' main aim is said to be the politicization of villagers in preparation for popular uprisings to coincide with Fretilin attacks.

Ramos Horta, who represents Fretilin at the United Nations, denied charges that Fretilin is a leftist movement. "Fretilin embraces shades of political thought from the conservative right to communism, but the dominant tendency is social democrat," he said.

Ramos Horta claims the annexation of Timor has caused damaging internal conflicts in the Indonesian armed forces. He said an Indonesian Air Force captain in charge of security in the east of the island had obtained political asylum in France in March of last year when he accompanied Defense Minister Mohammed Jusuf on a visit to Paris. He alledgedly gave his motive as opposition to Jakarta's policies in East Timor.

In 1981 two Indonesian battalions reportedly refused to take part in a sweep against guerrillas in the island's interior, laying down their arms for Fretilin to pick up and returning to the capital, Dili, empty-handed.

The guerrillas say their most pressing problem is a shortage of arms and ammunition.