The Human Rights Commission of Sri Lanka ruled police used excessive force in dispersing undergraduates, with at least nine students taken to hospital following a protest in Colombo in October.
"(The) police action is a violation of the universal declaration of human rights as well as the international covenant on civil and political rights," the independent commission said in a 32-page report.
It ordered the police to pay compensation to nine students hospitalised following the October 29 protest when officers launched water canon and teargas in addition to a brutal baton charge that was captured on television.
The students had demanded an accounting course was recognised on a par with other degrees, a demand the government later agreed to.
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The commission heard evidence that fleeing students were pursued by officers who hit them on the head and back. Even female students who had fallen during a stampede were targeted.
Official sources said that it was the first ruling against the police -- by the commission or any other body -- specifically referring to international humanitarian law in addition to national constitutional guarantees of freedom of association and equality before the law.
Sirisena came to power promising to end a culture of impunity for rights abuses by the police and security forces as well as corruption and nepotism under former strongman Mahinda Rajapakse.
Sirisena has also promised to investigate allegations that troops killed tens of thousands of Tamil civilians in the final months of crushing Tamil rebels in 2009 under his predecessor.