Prime Minister Indira Gandhi's government has been accused in Parliament of spying on opposition politicians by intercepting their mail under an old British imperial law that was used against leaders of India's independence struggle.

The government has not denied the charge, which raises the specter of Gandhi's 21 months of authoritarian emergency rule before she was carefully cultivated image as a free, democratic state.

The charge arose in both houses of Parliament Monday after the authoritative magazine India Today said police officers were intercepting and reading the mail of 264 persons in Delhi alone. The list included opposition political figures, Indian journalists who have been critical of the Gandhi government, businessmen and lawyers.

Among those named in the list, printed as a "roll of honor" by India Today, are the former foreign secretary, Jagat Mehta, a retired civil servant; Atal Bihari Vajpayee, foreign minister in the previous government and present leader of a major opposition party, and two editors and the owner of the Indian Express, a newspaper that has written the most critcal articles on the Gandhi government.

In the upper house of Parliament, Lal K. Advani, information minister in the former government, charged that his privileges as a member of Parliament were being interfered with by the government interception of his mail.

Advani said police in the post office even mixed up letters, putting them in the wrong envelopes after they were read.

"What amazes me," Advani said, "is the brazen manner in which this action is being defended by government spokesmen."

He has called for the censure of C. M. Stephen, Gandhi's communications minister, for telling Parliament that intercepting mail under certain conditions is legal. Stephen cited the Indian Postal Act of 1892, passed when as a law after this country gained independence 34 years ago.

The law was used by British authorities to quell incipient Indian rebellion against their rule. Then, as now, it allows the government to intercept the mail of any citizen "on the occurrence of any public emergency or in the interests of public safety or tranquility."

According to India Today, this law was invoked in May be Delhi police authorities against a range of people who it said constitute "a who's who of Mrs. Gandhi's opponents."

Included on the list are Vajpayee, Advanti, Vijay Kumar Malhotra and Madan Lal Khurana, all of the opposition Bharatiya Janata Party; Charan Singh, former prime minister; George Fernandes, a member of Parliament and labor minister in the government of Morarji Desai, and the three persons from the Indian Express newspaper: S. Nihal Singh, the editor-in-chief, Arun Shourie, the executive editor and owner Ram Nath Goenka.

Although reports have appeared in the press here in the past of Indian intelligence activities aimed at diplomats and foreign correspondents, none of the current revelaions has gone beyond the intercepting f mail of Indian political figures and journalists.