Police in the city of Leicester today charged four Asian men with conspiring to assassinate Indian Prime Minister Rajiv Gandhi during his two-day official visit here that ended last night.

The police did not identify the men and gave no details of the charges.

The four, who were said to be residents of Leicester, were among 10 persons arrested there last Friday and held without charge under Britain's Prevention of Terrorism Act.

Leicester police would not say whether the four being charged are Sikhs, and a spokesman said "we are not interested in their religious persuasion."

Some militant Sikh leaders here, who advocate a separate Sikh state in the Indian state of Punjab, openly applauded the assassination last year of Prime Minister Indira Gandhi by two of her Sikh bodyguards.

Heavy security surrounded the visit to London of her son and successor, Rajiv Gandhi, and the trip passed without reported incident.

Gandhi, who arrived here Monday morning, has charged Britain with lax treatment of Sikh extremists whom India believes to be terrorists. Before Gandhi left here last night for the Commonwealth Conference in the Bahamas, Prime Minister Margaret Thatcher agreed to extend Britain's 1978 Suppression of Terrorism Law to include India.

The change in the law would effectively obligate Britain to extradite Sikhs accused in India of terrorist acts, even if they claimed the acts were of a political nature.

United Press International reported that suspected Sikh extremists killed a local leader of Gandhi's Congress (I) party Wednesday, the first slaying of a politician since last month's elections in Punjab.

Ram Lubbaya, 50, was gunned down in Tarn Taran, 25 miles southwest of the Sikh holy city of Amritsar, as he waited on customers in his dairy shop just before dawn, officials said.