From Westbrook, Maine, to Los Feliz, Calif., from Beanville, Mont., to Jay, Fla., upwards of 80,000 people got together last night at 2,800 grass-roots parties to launch Jimmy Carter's 1980 presidential offensive, officially announced in a somber speech only hours earlier.

For five minutes they watched him on CBS network and cable television, in a prime-time spot sandwiched between "California Fever" and "Hawaii Five-O."

The spot cost the Carter campaign $60,000, but the evening's small parties were expected to raise an estimated $125,000 from committed and uncommitted guests.

The night was the culmination of two months' work and as a California host, admittedly biased, put it this week, "one of the greatest organizational devices I've ever seen."

In the big cities, there were bigger parties -- and bigger givers. At the Washington Hilton, about 1,700 people at $500 a head turned up last night for a mob-scene dinner, and throughout the next week, 11 similar events are scheduled in New York, Atlanta, Chicago and other cities.

The goal of the offensive, strengthened by new polls showing the president and challenger Ted Kennedy running neck and neck, is to raise $2 million by the end of the year.

That would bring the Carter people their desired $5 million for 1979, and should ensure that the campaign receives a very roughly estimated 40 percent of that -- approximately $2 million -- in federal matching funds for this year.

"When we conceptualized the week," says Tim Finchem, national fund-raising director, "we wanted to take as much advantage as possible of the president's announcment."

That meant a strategy of saturation: a dozen major fund-raising dinners around the country, the series of small-giver get-togethers, a direct-mail solicitation to 300,000 households and a half-hour television documentary on Carter's presidency designed to tie everything neatly together, then conclude with a public appeal for support.

Except that Iran kept intruding. By last weekend, the 2,800 hosts were sent Mailgrams explaining the half-hour documentary on the president would be delayed indefinitely. Instead, Carter would speak for five mintues. And by yesterday afternoon, his planned personal appearance at last night's black-tie dinner in Washington had been scrubbed entirely.

Otherwise, the systematic contacts to small-town hosts went off like clockwork. Patterned after 1976, they focused on letters:

A personally signed thank-you from Rosalynn in early November;

A how-to-give-a-party memo from regional headquarters a few days later;

Another thank-you from national headquarters plus a packet of bumper stickers, campaign buttons, souvenir tickets and issue information;

A thank-you mailgram on Nov. 30 from campaign chairman Robert Strauss that also advised of Carters decision to curtail political activity and to dealy the television special;

A personally signed thank-you letter from Carter dated Nov. 26;

A thank-you Mailgram from both Jimmy and Rosalynn dated Dec. 1, asking hosts to find a friend to give another party.

"Whether it's Auto-pen or not," says Mike Galizio of Los Feliz, administrative assistant to a California assemblyman, "it's still a big deal for the common guy who gets a letter from the president." Galizio was having his framed.

There were no written follow-ups to the Washington dinner, which was the week's flagship event. Rather, about 600 of those 4,500 who were sent invitations to this dinner got phone calls. And who were the select few?

"The same old crowd" -- or, "the money people" -- explained Nate Landow, the multimillionaire developer who, after a lunch two weeks ago with Carterites Tim Kraft and Evan Dobelle at Duke Zeibert's, emerged as the dinner chairman.

"I really didn't know what I was getting myself into," he said yesterday, hiding out from the surrounding bedlam in the foyer of the Hilton's Health Club.

Landow compiled most of the dinner-guest list himself, drawing from rosters of Fortune 500 corporations, Democratic National Committee people, Carter-Mondale supporters, trade representatives as well as "my constituency -- the Washington business community."

He also picked the menu (the omnipresent fruit cup and filets of beef), the entertainment (the Duke Ellington band and Gene Donati orchestra) and did, by his own calculations, 75 percent of the seating arrangements. He placed himself at the president's table, should the president show up.

In other parts of the country, the smaller dinner arrangements were on a more modest scale, but no less significant.

Sandy Plowman of Dallas, a securities analyst, said she had invited "a diverse group of young and not-so-young professionals reflective of people at the grass roots who supported Carter four years ago.

"Most weren't sure how they felt about him, you know, the kind who say 'I think I'm for Jimmy Carter but I'm not sure and I want more information.' So I intend to have 30 people arrive who are marginal and, when they leave, 30 people who aren't."

San Francisco stockbroker Philip Schaefer said his phone started ringing last Wednesday night even before Carter concluded his nationally televised press conference.

"Some people had been neutral or leaning to Kennedy," said Schaefer, "but they said Kennedy had been disappointing and Carter was looking awfully good."

In Westbrook, Maine, Mayor William O'Gara said he and his wife Beverly planned nothing fancy and didn't intend to solicit contributions.

"We just asked people if they were committed to anyone," said O'Gara. "Hopefully we'll get to talk over some things because we want to start developing good people now who will go to Maine's February caucus."

Carroll T. Berry of South Burlington, Vt., a retired foreign service officer, said it was "kind of early in the game to ask people to commit themselves so I just said, 'Come and let's just talk about the issues.'"

In Washington, law student Joel Odum put a Carter campaign poster on his Mt. Pleasant front door and said his party was for "all those people who aren't going to the Washington Hilton.

"At first I thought we shouldn't be celebrating, what with the crisis in Iran," said Odum, "but I guess I was wrong because everybody's been calling to say they wanted to come."

The party-givers are all part of the highly organized national network of names that Carter organizers court to build their support.

Schaefer, for instance, was a guest at the White House dinner celebrating the signing of the Arab-Israeli peace treaty, while O'Gara and Berry were in a group of New England leaders briefed about energy and Iran at the White House last week.

"The people who made it possible for Carter to be elected in 1976 weren't necessarily inside the campaign structure," according to Jack Watson, an assistant to the president. "But they were excited, and with those who were inside the structure became the same grass-roots network that is going to be apparent across this country next year."

As Berry put it: "My name's in the computer."