LONDON (AP) _ Amnesty International on Wednesday urged the Indian government to investigate what it called increasingly credible charges that Indian police have killed scores of Sikh activists in staged encounters.

In a review of human rights violations in India in the past 18 months, the London-based human rights organization also complained that an investigation into an alleged police massacre of 80 Moslem villagers has been kept secret.

Amnesty also said in the 26-page report on India - the world's largest democracy - that it has received reports of dozens of prisoners dying in police custody across the country after being tortured.

Amnesty blamed police excesses, arbitrary arrest and prolonged detention without trial on the removal of key legal safeguards in security laws and ''official failure to clamp down on lawlessness by India's national and state security forces.''

''Many victims were involved in violence themselves and have killed civilians and security personnel, notably in Punjab,'' Amnesty said in a statement accompanying the review. ''But action taken against them has transgressed international human rights standards.''

The report was sent to to Indian Prime Minister Rajiv Gandhi in June, but he did not reply, Amnesty said.

Gandhi, who would not allow Amnesty International to visit the Sikh- dominated northern state of Punjab, has denied reports of torture by police and security forces.

The report said most of the victims were political activists or members of poor, underprivileged groups.

The report said Amnesty was told by a Punjab civil rights group, which it did not identify, that security forces killed more than 70 Sikhs in fake or staged encounters between May 12 and Aug. 22 last year in the Amritsar district, where Sikh radicals are demanding a homeland.

Amnesty said the group claimed another 50 Sikhs were killed in similar encounters in Punjab's Faridkot district between May 12 and July 29, and identified 29 of the alleged victims.

''There certainly has been too great a pattern of 'encounter' killings over the years for it to be dismissed as the odd, isolated case,'' said Amnesty International spokesman Sean Stiles.

Similar killings by military and police forces have been reported in Bihar, Andhra Pradesh and Manipur states, the reprot said, with security forces having shoot-on-sight powers granted in 1986, the report said.

''They (the reports of killings) have gained in credibility through circumstantial evidence,'' the report said.

On the alleged massacre of 80 Moslems, Amnesty said a civil rights group claimed to have detailed evidence that police carried out the killings, including of children as young as 3, during Hindu-Moslem clashes in Uttar Pradesh in May 1987.

The report said witnesses testified that the Provincial Armed Constabulary dumped bodies in rivers and canals or burned them.

Authorities in Uttar Pradesh, where Moslems are a 15 percent minority, have denied there was a police massacre.

Hundreds of political detainees have been imprisoned for four years under special security laws, including 326 Sikhs held in Jodhpur jail since 1984, the report said.