LIMA, PERU -- After nearly eight years as Latin America's most secretive and ruthless guerrilla group, Peru's Shining Path has begun explaining its philosophy and strategies in public.

Discussion papers detailing the movement's ideas and plans were published last month. A lengthy report disclosing the group's "first national party congress" went on sale last Sunday.

El Diario, a low-budget, Lima-based newspaper, has circulated Shining Path documents, providing a daily look at the movement's thinking. Meanwhile, imprisoned insurgents who once shunned interviews now courteously receive visitors at Canto Grande, the capital's maximum-security prison, for talks beneath painted slogans and revolutionary posters.

Other guerrilla groups have practiced public relations to make their struggles known. But the Shining Path had thrived on secrecy. Predictably, its new propaganda offensive has generated speculation.

Some view the campaign as a natural outgrowth of the movement's expansion. By publicizing its program, Shining Path's leadership appears to be trying to strengthen coordination of far-flung ranks and facilitate recruitment.

Conversely, others suspect that the new outspokenness may be a sign of some weakness, perhaps an attempt to bolster morale following reported losses, or the result of a divisive debate over aims and methods bubbling to the surface from within Sendero Luminoso, as the group is called in Spanish.

Generally, though, the parting of Sendero's veil is regarded as part of a drive by the guerrillas to become a political force, not just in the countryside but in Peruvian cities.

Despite governmental efforts to confine them to their original mountain base in Ayacucho Province, the insurgents have expanded along the Andes, dipping into the coca-rich jungles of the Upper Huallaga Valley and pushing into the coastal capital of Lima.

Sendero's recent documents acknowledge a strategy more broadly based than its initial plan, inspired by the strategies of Mao Tse-tung, to "encircle the cities from the countryside."

While rural areas remain the principal theater of combat, the guerrillas have begun to stress urban subversion as a complementary activity and are attempting to infiltrate unions, student organizations and leftist political parties. Bands of apparent Senderistas tried to dominate a rally in Lima last month organized by the country's main union federation during a one-day general strike.

In targeting cities, Sendero is clearly trying to capitalize on one of the stark demographic facts of Peru -- the vast migration of hundreds of thousands of peasants from Andean towns to coastal slums. These transplanted villagers have swelled the capital's population to 6 million people -- 10 times the number three decades ago -- and constitute an often alienated and potentially explosive political force.

But Sendero's bid for prominence in the shantytowns is not so easy for a guerrilla movement accustomed to the isolated reaches of the Andes. For one thing, teeming, cramped urban settings demand different tactics.

For another, in contrast to the political vacuum of the deserted sierra, at least half a dozen radicalized leftist parties already are vying for influence in the shantytowns. They do not welcome competition from Sendero, nor do they approve of its fanaticism. At a "national popular assembly" in November, the parties issued a communique criticizing "the sectarianism and militarism of Shining Path."

"To be effective in the cities, Sendero is going to have to modify its program and style," said Raul Gonzalez, who studies the movement. "It has already begun to do this by saying, for instance, that its revolution will include a place for the middle bourgeoisie."

Sendero continues to be the fiercest force militarily on the Peruvian left. But politically, the pro-Cuban Tupac Amaru Revolutionary Movement -- the other guerrilla group -- is more popular among leftist activists. Led by middle-class urban intellectuals, these revolutionaries have engaged more in publicity stunts and night attacks on foreign embassies and business interests than in murders of police or civilians. Styling themselves after traditional Latin American communist insurgencies, they have cultivated a Robin Hood image.

Last November, the group ventured out of Lima, where it has been operating since 1984. It launched a "rural column," taking over several towns in the northwestern jungle state of San Martin. The government responded by declaring the area an emergency zone and sending in Army troops.

The group also may have contributed to Sendero's decision to try a few publicity ploys of its own, including the announcement of a congress. Although the exact date, place and manner of the meeting were not given, the event was hailed as Sendero's most important political happening since the group began armed action in 1980 to overthrow a system it describes as "semi-feudal, semi-colonial."

Hints of a Sendero opening began to appear in late 1986 as the group sought to recover from the massacre of about 250 accused terrorists during uprisings at three Lima prisons. The insurgents published a 110-page document explaining their ideology and tactical positions.

But for all its measured new self-exposure, Sendero remains Latin America's most mysterious revolutionary movement. It combines the vision of a Chinese-style revolution with the tactics of a European terrorist group to exploit a distinctly Peruvian cultural clash between impoverished Quechua-speaking Indians and the descendants of Spanish and other European immigrants.

The group encourages fanatical loyalty among its followers and, as far as is known, continues to reject links with foreign revolutionary movements. It persists in "selective assassinations" of policemen, government development workers, peasant leaders, businessmen and ruling party officials. It goes on sabotaging bridges, electrical pylons and factories. The dynamiting this month of a freight train loaded with ore caused a loss of several million dollars and was attributed to Sendero.

At the same time, intelligence sources report a rise in desertions among rank-and-file guerrillas. The desertions are said to reflect disgruntlement with Sendero's aloof leadership or, in some instances, corruption stemming from the movement's reportedly growing involvement in the cocaine trade.

Police claim, too, to have broken up a number of Sendero columns in the countryside and clandestine cells in Lima. Deputy Interior Minister Augusto Mantilla now puts the number of Sendero combatants in rural areas at fewer than 500, but the armed forces and others estimate there may be as many as 5,000 full-time militants.