Republican Party Chairman Frank Fahrenkopf has written thousands of party stalwarts urging a mass Republican call-in to the Democrats' nationwide fund-raising telethon next weekend, a move Democratic Party Chairman Charles T. Manatt charged is a "dirty tricks" plan to "sabotage" the event by jamming the phone lines.

"Urgent--Urgent," Fahrenkopf's cablegram began. "Urgently need your help to combat Democrat smear campaign aimed at President Reagan and entire Republican Party." It warned: "Reports reveal entire Democrat telethon will use half-truths, slanted information and distorted man-on-the-street interviews to attack and insult President Reagan and Republicans in House and Senate."

And more: "Hollywood left-wingers likely to appear shoulder-to-shoulder with liberal kingpins such as Ted Kennedy and Walter Mondale."

The Democratic Party will conduct the 17-hour telethon over the NBC television network Friday and Saturday. It will include a mix of entertainment by stars, spiels by politicians and short partisan commercials critical of Reagan policies.

"Help me in an emergency campaign to counterattack the unfair Democrat telethon," Fahrenkopf wrote fellow Republicans. "Call the Democrat fund-raisers . . . . Tell them you support the president . . . ."

And also: "Rush an emergency contribution of $15 to the Republican National Committee right now . . ."

Manatt said that Democratic Party attorneys will consider filing an unfair practices complaint with the Federal Election Commission this week. "They're going back into the dirty tricks bag again," Manatt said. " . . . Obviously they are trying to jam our telephone lines and sabotage our fund-raising effort . . . . It is fitting that this Republican dirty trick comes in the same week that the Republican president pardoned a Watergate figure."

Manatt later added: "I'm shocked and appalled that Chairman Fahrenkopf would sign such a letter and I call on him and the president to retract and disavow this dirty trick."

A spokesman said Manatt plans to write the president asking him "to get the Republican Party under control."

The telethon effort has been much-troubled, provoking battles in the Democratic Party well before the Republicans made the controversy bipartisan. And despite Manatt's severe rhetoric, the Democrats greeted the Republican cablegram the way publishers react to news that their latest book has just been banned in Boston.

It was Democratic Party officials who made a copy of the Fahrenkopf letter available to The Washington Post. Democratic Party spokesman Bob Neuman explained that the Democrats had received their copy through an honorable trick: a Democratic insider had duly contributed money to the Republican committee long ago in order to get on the Grand Old Party's mailing list, he said.

Republican Party spokesman William I. Greener III said the RNC mailed the cablegram to thousands of GOP supporters but he denied that the effort to mount a Republican call-in was intended to sabotage the Democratic telethon.

"Not knowing the number of telephone lines the Democrats have, we don't know if it would even be possible to jam them," Greener said. "The Democrats were encouraging the public to call in and comment during their telethon. We felt it was only fair and proper to let some of our supporters know that they too could comment."

A 1982 Democratic telethon on cable television fell victim to jammed phone lines and half of the callers in the first hour were pro-Reagan, said Democratic spokesman Neuman, adding: "This letter seems to confirm our suspicions that our fund-raising effort last year was sabotaged." Responded the GOP's Greener: "I can categorically deny that. We didn't have a thing to do with it."