NEW DELHI, March 27 (Reuters)--Prime Minister Indira Gandhi today described the military imposition of martial law in East Pakistan as "not merely suppression of a movement, but meeting unarmed people with tanks."

"An opportunity has been lost in East Pakistan for Pakistan to strengthen herself," Mrs. Gandhi told cheering members of the Indian Parliament.

"The opportunity has been lost in a manner which is tragic and agonizing, about which we could not find strong enough words," Mrs. Gandhi said.

"The people of East Pakistan," she said, "spoke with almost one voice when they voted to send delegates of Sheik Mujibur Rahman's Awami League, now banned, to the National Assembly." The party was given a majority of the seats in the Assembly, which was to have met March 3 to draft a constitution.

Mrs. Gandhi's statement came after members had voiced dissatisfaction over a statement by the External affairs Minister, Swaran Singh. He had only expressed "great concern" over the developments in Pakistan's eastern wing, which borders several northeastern Indian States.

Mrs. Gandhi said that if her colleague had not spoken with passion, it was not for lack of feelings on his, or the Government's part, but because "we are deeply conscious of the historic moment when something new has happened in East Pakistan."

India and Pakistan were one country under British control. In 1947 they were partitioned creating Pakistan as a Moslem country. Ensuing frontier and religious battles between Indian Hindus and Pakistani Moslems led to hundreds of thousands of deaths. The countries, which went to war over Kashmir in 1965, are still antagonistic.

The Prime Minister said India had welcomed the Awami League's victory in the December general elections, not because she wanted to interfere with the country's internal affairs, "but because these were the values for which we have always stood for and for which we have spoken."