Gen. David Padilla, the ousted armed forces chief and an ex-president, has appealed to the Bolivian people to fight with "strength and courage" against the government of coup leader Col. Alberto Natusch Busch.

Padilla's "open letter" became known today despite tight censorship imposed since the coup last Thursday and subsequent repression of resistance that has resulted in up to 70 deaths.

In addition to the letter to his countrymen, Padilla has also written two letters in recent days to his former troops, asking them to lay down their arms instead of continuing to protect the Natursch regime in the face of widespread popular opposition.

Diplomatic observers here interpreted the three letters, which have not been mentioned on the rigidly controlled government radio and television, as another sign of opposition within the armed forces.

Handwritten on plain stationery, Padilla's letter to "the people of Bolivia" in effect urges them to continue the passive and armed resistance that began within hours of last Thursday's coup.

"Strength and courage brother Bolivians," Padilla wrote, "to defeat the adventurers and criminals who in the name of the country and the institutions of the armed froces are committing the worst atrocities and spilling blood."

The letter, dated yesterday, ends with the words: "Long live Boliva."

Until being fired by Natusch, Padilla had been instrumental in the Army's support for efforts to restore representative rule for Bolivia's 5 million mostly poor, largely Indian population.

Last night, President Natusch warned members of Congress of a wave of repression surpassing any in the modern history of this chronically unstable country if they did not accept his proposal to stay in the presidency at least until next year.

Natusch ousted the Andean nation's first civilian president in over a decade, Walter Guevara yarze, who had the support of the Carter administration and other democratic governments in the hemisphere.

Natusch issued his warning after the Congress, which he closed last week but allowed to meet in an emergency session yesterday, proposed that Natusch resign in favor of a civilian president to be elected by the Congress. The legislators met again today without result.

Most political and diplomatic observers here said today that the impasse seemed to end whatever hopes existed of a negotiated settlement.

"It now looks as though the only way Natusch will leave the presidential palace is horizontally," said one diplomat here. "He obviously has decided to try to tough it out."

Sen. Jose Luis Roca, one of the members of Congress who met with Natusch last night, said he and other members of the negotiating team took Natusch's treat of repression seriously. Many members of Congress, after the emergency session last night, slept in hotels or at the homes of friends.

Most unin and many political leaders are already in hiding and there were reports that soldiers arrested 20 union leaders last night. These repor reportedly included Liber Forti, the personal secretary to Bolivia's most prominent labor leader, Juan Lechin, and Simon Reyes, a mining union leader and Communist member of the House of Deputies. Lechin went underground Saturday.

Natusch has alredy imposed martial law on the country, placed it under a state of siege, prohibited meetings of more than three people, declared press censorship, closed down several foreign news agencies and ordered La Paz occupied by troops.

The most effective weapon used against the new government, which appears to have almost no popular support, has been a general strike that has virtually paralyzed the country for the past six days.

The strike, originally called by Lechin's Workers' Central of Bolivia, has continued on its own momentum nationwide because of widespread opposition to the Natusch government.

Almost the only movement here is by foot. Public transportation has stopped, in part because taxi, bus and truck drivers have joined in the protest and in part because students and workers have bult barricades on main streets.

Although troops began clearing away barricades in the center of La Paz today, two busloads of airline workers trying to reach El Alto airport were turned back. Another bus was fired on by antigovernment brigades that control working-class neighborhoods along the road to the airport. About 300 foreign tourists were evacuated today.