The four students were arrested by Sri Lanka's anti-terror force for honouring dead Tamil Tiger leaders by marking the LTTE's Heroes Day on November 27.
The arrests came after a crackdown in the Tamil-dominated northern city of Jaffna where plans were afoot to honour the Tamil 'martyrs'.
The four Jaffna University students were initially detained in Vavuniya and later sent for a 'rehabilitation' programme to a centre at Welikanda, which holds an estimated 600 alleged former LTTE fighters.
Human Rights Watch said the students should either be charged for their 'crime' or be released without delay.
"Arresting four students without charge and sending them off for 'rehabilitation' sends a dangerous message that any Tamil can be detained arbitrarily and indefinitely," said Brad Adams, Asia director at Human Rights Watch.
"The Sri Lankan authorities should realise that such actions generate legitimate grievances, not reconciliation," he said.
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The students' parents who were allowed to meet them at the rehabilitation centre said the students were told that they will be detained until they have completed a 3-month lecture series on the theme of "reconciliation".
The parents had also claimed that the students were being interrogated, with efforts to intimidate them into providing information about other students.
According to HRW, Sri Lankan authorities consider celebrating LTTE Heroes Day as tantamount to supporting terrorism.
It said this year, security forces had arrested at least 10 other Tamil activists and university students in relation to an alleged arson attack on November 28 supposedly linked to commemorations.
"The Sri Lankan government needs to recognise that engaging in peaceful activities that conflict with the government's views is an exercise of basic rights, not a criminal offense," Adams said.
"The four students should be promptly released unless the government has evidence they violated the law and charges them," he said. PTI
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