Despite occasional downpours, Willie Nelson, Bob Dylan, Neil Young and 50 other leading rock and country musicians came together today to help the nation 's farmers in a concert that was both a musical and political event.

In organizing the 14-hour benefit concert, known as Farm Aid, Mr. Nelson sought to help the nation's financially pressed farmers by raising money and bringing their plight to the nation's attention. The concert was modeled after last July's Live Aid concert, which raised money for famine relief.

"I'm a farm boy from Oklahoma," said Roger Miller, the popular country singer who performed with Mr. Nelson. "We musicians don't have all the answers to the farm problem. We're just trying to bring attention to it."

Wild Cheer Despite Rain

About 78,000 people paid $17.50 a ticket to crowd into Memorial Stadium at the University of Illinois to hear the performers, who also included Joni Mitchell, Billy Joel, Bonnie Raitt, Merle Haggard, John Denver, Glen Campbell and Charlie Daniels. The crowd, many of them students at the University of Illinois, covered themselves with tarpaulins in the school colors, orange and blue, and cheered wildly as the musicians performed despite the rain.

The concert was held in a week when debate about omnibus farm aid bills is expected to reach a climax in Washington. Senator Tom Harkin, an Iowa Democrat who is sponsoring one of the bills, and who attended the concert, predicted that it would spur the passage of measures aiding farmers. Speaking Saturday on his way to the event, he said it would "focus attention on the fact that the old ways of helping farmers are not working."

Goal Was $50 Million

After Mr. Nelson announced the concert Aug. 17, its organizers said they hoped to raise $50 million. As evening neared today, however, the Farm Aid telethon, which was averaging 30,000 calls an hour, had raised $2 million. They said contributions were coming in at the rate of more than $500,000 an hour. By mid-evening Farm Aid had reportedly collected $7 million.

Concert organizers said Mr. Nelson would decide how the money would be spent. Last week he said he wanted the Farm Aid funds to be used for a hot line and legal assistance for troubled farmers and for retraining and job counseling for farmers forced to give up their farms. He also said some of the money should be used to increase the public's awareness of farm problems. Mr. Nelson said no money would be spent on lobbying. Some of the money, he said, should go directly to needy farmers.

"I heard a wise man say you can spend quickly or spend it right," said Mr. Nelson, a long-haired, bearded Texan. "I am not going to rush into anything." He said he had a few recipients in mind, but he would not identify them.

Corporation to Handle Funds

The money collected was being deposited in the First National Bank of Champaign. Farm Aid Inc., a nonprofit corporation, has been set up in Illinois to handle the funds, which will be audited by Price Waterhouse, the accounting firm.

Working with a host of farm groups, Mr. Nelson set up a telephone number (1-800-FARM-AID) in Omaha to collect money for a full year.

Farm Aid's organizers conceded that the concert would not raise enough money to solve the problems relating to the nation's farmers' total debt of more than $210 billion.

"The amount of money collected will be insignificant for individual farmers, " said Elmer Kunkel, a 51-year-old corn and soybean farmer from Iowa who came to the concert with 500 other farmers on a chartered train. "The most important thing is that the concert has brought us together, and it has brought the media here to draw attention to the facts."

The concert attracted 1,200 journalists, including some from Italy and Australia.

Many of the musicians said they wanted to lend moral support to farmers because they came from farm families or farm areas. "My grandfather lost a lot of farms during the Depression," said Arlo Guthrie, the folk singer. "The concert is a wonderful thing because it gets the nation to stand up and say, 'We 're behind you farmers.' "

Several of the performers urged farmers to contact legislators to seek support for particular measures. Performers also criticized proposals by the Reagan Administration that would cut farm subsidies to reduce the Federal deficit.

The performers, who also included Kenny Rogers, B. B. King, the Beach Boys and John Cougar Mellencamp, flew in at their own expense and performed free.

"We've been working to publicize the farmer's plight since 1979 with the tractorcade to Washington," said Ken Crego, a 31-year-old farmer from Qulin, Mo., who drove 350 miles to the concert with a caravan of other farmers to highlight their cause.

The Nashville Network, a country and western cable network, beamed the concert live into 24 million homes. Local television stations bought syndication rights to broadcast three hours of Farm Aid into another 65 million homes tonight. In addition, concert organizers said the Voice of America was broadcasting parts of the concert around the world.

Hardship for Farmers

According to some estimates, including that of agricultural economists at the University of Illinois, one-fourth of the nation's 650,000 full-time farmers -there are another 1.7 million part-time farmers - are under severe financial stress. These experts estimate that as many as 65,000 of the full-time farmers are so deeply in debt that they face loss of their farms within two years.

The seeds of today's farm crisis were sown in the 1970's. Farm leaders, farm lenders and Government officials urged farmers to plant "from fence to fence" to help turn the nation's Farm Belt into the world's breadbasket. Encouraged by high grain prices, many farmers borrowed heavily to increase their acreage.

But many farmers soon found themselves in trouble as interest rates skyrocketed. Making matters worse, several restrictions on grain sales caused America's customers to look elsewhere for their grain needs. And the latest problem is the strong dollar, which helps make American grain exports more expensive than those of other countries.