COLOMBO, Dec 10 (Reuters) - Human rights abuses have made Sri Lanka one of the most lawless places in the world and the government must do more to investigate them, a regional human rights group said on Monday.

The Hong Kong-based Asian Human Rights Commission complained of what it called "the constant killing" of people in police detention and the falsification of information to "justify crimes committed by state agencies"."

Describing Sri Lanka's criminal justice system as "dysfunctional", the group said "As long as this situation remains, life will remain a nightmare to all civilians in the country."

It said in a statement to mark International Human Rights Day the situation was worsened by the targeting of civilians by Tamil Tiger separatist rebels, the government and paramilitary groups. It said criminals were also profiting from the climate.

President Mahinda Rajapaksa's government says security forces have not been involved in abuses and that a presidential commission is probing allegations, but the government has rejected calls for a U.N. human rights monitoring mission.

"It is our aim to help our people to enjoy all freedoms of democracy with respect for human rights, free of the fear and oppression of terrorism," Rajapaksa said in a statement.

The Tigers, who seek an independent state in north and east Sri Lanka, accused the government of abusing human rights and the international community of turning a blind eye.

"The world is standing by and just watching these gruesome death squad activities of the Sri Lanka State," said Selvy Navaruban, Tamil Tiger spokeswoman on humanitarian issues and human rights in an email statement.

Nearly 70,000 people have been killed in the conflict in Sri Lanka since 1983 -- around 4,500 in the last year alone. (Reporting by Ranga Sirilal; Editing by Matthew Tostevin)

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