COLOMBO, Dec 4 (Reuters) - Amnesty International criticised Sri Lanka's government on Tuesday for violating the human rights of thousands of Tamils who were arrested days after two bombs exploded in the capital Colombo.

The bombs, which killed 20 people last Wednesday, have been blamed on Tamil Tiger rebels -- an organisation outlawed by many countries and held responsible for hundreds of deaths in their fight for an independent state in north and east Sri Lanka.

Sri Lanka's government denied infringing the rights of those arrested and said the move was a necessary security measure. More than 2000 people were held following a search operation around Colombo and the north-western district of Puttalam.

"Those arrested may be detained in inhumane conditions; denied access to lawyers, courts and family members and face the risk of torture, other cruel, inhuman or degrading treatment or punishment and prolonged arbitrary detention," a South Asian researcher for rights group Amnesty International said in a statement.

Jeyaraj Fernandopulle, minister of Highways and Road Development, denied that the arrests targeted any particular ethnic group, and said most had been released.

"I must tell clearly that this is not against any nationality. Not only Tamils, but Sinhalese and Muslims too were arrested," Fernandopulle said, adding that 2,300 people had been released and only 202 remained in custody.

In June Sri Lanka's Supreme Court ordered the government to stop evicting minority ethnic Tamils from the capital a day after police deported hundreds to the island's war-torn north saying they were in capital without a valid reason.

The latest moves come after a female suicide bomber blew herself up in Colombo on Wednesday, killing a minister's aide, and a parcel bomb killed 19 people in a shopping centre in a suburb hours later.

More than 5,000 people have been killed in fighting between the military and the Liberation Tigers of Tamil Eelam (LTTE) fighters since early 2006 alone, taking the death toll since the war erupted in 1983 to around 70,000.

Military analysts say there is no clear winner on the horizon, and fear the war could grind on for years. (Reporting by Ranga Sirilal; editing by Sophie Walker)

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