A group of liberal members of Congress, flanked by actor Cliff Robertson and international relief officials, announced yesterday that they will seek more than $1 billion in additional aid for famine-stricken Ethiopia and the rest of Africa when Congress convenes next month.

"The money may seem like a large amount, but it's less than the cost of one Trident submarine or four MX missiles," Rep. Mickey Leland (D-Tex.) said at a Capitol Hill news conference. He later acknowledged that the legislation would face an uphill fight "at a time when we are in an austerity program."

Rep. Ted Weiss (D-N.Y.), who also will sponsor the bill, said that "in contrast to what we spend to do harm and violence to people, this is a small amount."

Robertson added his voice to the call for increased aid and expressed concern that the public might fall victim to "compassion fatigue" and lose interest in Ethiopia, where, according to the United Nations, famine has claimed more than 300,000 lives.

The actor, who just returned from Ethiopia, said he had visited Biafra on a similar mission in 1968 and that "I thought I was girded for what I would see. I wasn't . . . . It was far worse than anything I could imagine."

He said he met a man who had started out for a feeding center with his wife, three children and a plow, but only the father survived. "When he got there, all he had was the plow in his hand," Robertson said.

He described walking down rows of huts, seeing perhaps 300 families lying along the road and looking inside to find "little skeletons, 3 months old or 6 months old. Multiply that hundreds of times and you understand the scope of the problem," Robertson said.

The group also launched a drive for public contributions to supply food, medicine and other relief to African countries that are suffering from a crop-killing drought. That campaign will be coordinated by InterAction, a New York-based association of more than 120 private agencies that help provide relief for Third World countries.

About $787 million of the $1 billion aid proposal would be added to the U.S. Food for Peace program as a supplement to the program's fiscal 1985 budget, Weiss said.

Another $225 million would go to projects that promote longer-term recovery, Weiss said. These funds would consist of $80 million for rehabilitation programs, such as agricultural development and food preservation projects; $70 million for international disaster assistance, including medicines and vaccines; $50 million for refugee assistance and $25 million for overcoming transportation and other problems of continuing food aid programs.

The bill would also would include $4 million for improved planning, monitoring and supervision of the food aid, Weiss said.

Sen.-elect Paul Simon (D-Ill.) and Sen. John Melcher (D-Mont.) will sponsor the bill in the Senate.

"Only one year ago," said Melcher, "the U.S. distributed $12 billion to farmers, paying them to idle 80 million acres of farmland." Still, Melcher said, the farmers have produced overflow crops and he urged the Reagan administration to send to Africa what he said are huge reserves of wheat and dairy products stored in federal warehouses.

According to Weiss, the administration has sent food aid worth more than $300 million to Africa just in the past two months.

But a spokesman for the House Select Committee on Hunger said the total pledges of food from all nations, including the United States, is less than one-fourth of what Ethiopia will need in the next year.

At the news conference, Peter J. Davies, the president of InterAction, said there had already been "a great humanitarian response to the crying out in Africa."

Contributions have come in from all over the country, Davies said, including money from a elderly man who wrote that he was living on disability payments, $5 from a convict in a penitentiary and $1.73 from a little boy, who taped the money to a piece of paper and printed: "My name is Scotty -- I'm 5 years old. Use this money to buy some food."