LONDON (AP) _ Amnesty International said Tuesday it had new evidence of political prisoners in Singapore undergoing torture while being interrogated by officials of the internal security department.

A 15-page report from the worldwide human rights organization said some of the seven prisoners - three women and four men - were kept without sleep by being subjected to round-the-clock questioning in cold rooms, doused with cold water and repeatedly slapped in the face if they were uncooperative in making self-incriminating ''confessions.''

Relatives who saw the prisoners said most of them ''looked extremely tired and haggard, with dark rings under their eyes, sore lips, coughs, and most appeared thinner,'' the report said.

It said relatives feared the prisoners might have been drugged or otherwise ill-treated because most had unnaturally darker and patchy skin complexions, despite having been held in windowless cells for 10 days or more.

Londo-based Amnesty said it was asking the Singapore government for an independent inquiry to investigate the allegations. It called for the prosecution of any offending officers and for the prisoners' release.

The seven were among a group arrested in mid-1987, accused of a ''Marxist conspiracy'' to overthrow the state, but later set free. Amnesty said the seven were rearrested in April after making a news statement denying the allegations and claiming they were ill-treated in detention.

After sending a fact-finding mission to Singapore in June 1987, Amnesty published a report in last October charging that the 22 people originally detained underwent ''severely cruel, inhuman and degrading treatment'' during interrogation.

All 22 were adopted as ''prisoners of conscience,'' the Amnesty term for detainees arrested for holding non-violent beliefs.

''We are concerned about the continued detention of these seven persons and we received via relatives and lawyers information from the detainees themselves for this new report,'' said Amnesty spokeswoman Carline Windall.

The women were identified as Teo Soh Lung, 39, lawyer; Chng Suan Tze, 39, lecturer in English, and Wong Souk Yee, 30, market research executive. The men are Vincent Cheng Kim Chuan, 41, theologian; Kenneth Tsang Chi Seng, 35, media analyst; Kevin Desmond De Souza, 27, student organizer, and Chew Kheng Chuan, 30, businessman.

Amnesty said it sent to Singapore authorities the names of officers who it had been told ill-treated detainees.