Union Carbide Chairman Warren M. Anderson left here today on a chartered jet for the United States as American lawyers began arriving to gather evidence for lawsuits that are expected to demand billions of dollars in compensation for victims of the Bhopal poison gas disaster.

Melvin Belli, the prominent California torts lawyer, was scheduled to arrive in New Delhi early Monday to gather evidence for a $15 billion lawsuit that already has been filed in the United States against Union Carbide Corp. The corporation's subsidiary, Union Carbide India Ltd., operates the pesticide plant in Bhopal that spewed out tons of poisonous methyl isocyanate gas into the adjacent slum neighborhoods, killing 2,000 or more persons.

The U.S. lawyers who are already in Bhopal said that the claims for compensation could total $400,000 to $500,000 for the clients they represent but that class-action lawsuits would push the figure into the billions of dollars.

John Coale, who said he had been interested in pursuing damage claims on behalf of several Indians, arrived with attorney Arthur Lewis. Coale is with the Washington firm of Coale and Associates. Coale and Lewis represented U.S. hostages held in Tehran in 1980 in suits against the Iranian and U.S. governments.

Meanwhile, Arjun Singh, the chief minister of Madhya Pradesh State, where the pesticide plant is situated, said he will file a compensation lawsuit against Union Carbide on behalf of the victims and their families. Singh returned to Bhopal today after visiting New Delhi for consultations with Attorney General L.N. Singh.

United Press International reported:

Union Carbide officials at company headquarters in Danbury, Conn., said Anderson would hold a news conference at 1 p.m. Monday at the Danbury Hilton Hotel.

Union Carbide's Indian subsidiary announced that it was contributing $800,000 to the state relief fund for victims.

The company also announced that it would open an orphanage in Bhopal for children whose parents died in the catastrophe.