The New York-based body said Sri Lanka's ending of a decades-long state of emergency last week would have little effect as separate anti-terror laws still allowed people to be detained for long periods without trial.
About 6,000 people are being held under the Prevention of Terrorism Act (PTA), the group said, two years after the end of the island's civil war.
"The Sri Lankan government announced that the state of emergency is over, but it is holding on to the same draconian powers it had during the war," Brad Adams, Human Rights Watch (HRW) Asia director, said.
"The government should repeal all its abusive detention laws and make all laws and regulations related to detention public," he said.
Sri Lanka first introduced emergency laws in 1983 when the Tamil Tiger rebels escalated their bloody campaign for an independent state for their ethnic minority.
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It ended with the defeat of the Tigers in May 2009.
The laws, which gave security forces sweeping powers of arrest, were