Minelle Fernandez, Al Jazeera's correspondent in Colombo, said authorities believe the latest measure is necessary.
"Increased security has undoubtedly prevented attacks in Colombo in recent times, but most Tamil parties and the people they represent feel this latest exercise unfairly targets an already suffering people," she said.
R. Yogarajan, the vice president of Ceylon Workers Congress, told Al Jazeera: "Whoever comes in has been registering but to ask them to come on a mass scale like this and to do it within eight hours is an impossible task and people are very scared as to why they are being summoned by the police in this manner".
"We had to give the address where we live, family details and the date we came to Colombo and when we are going back, " said M. Balakrishnan, a retired Tamil officer who was among those who had to register.
"They also asked what my children were doing," he said.
Paikiasothy Saravanamuttu, an analyst from the Centre for Policy Alternatives, said: "At the end of the day, you are only instilling some sense of second-class citizenship and deepening a perception of discrimination".
The registration order came as the military is intensifying an offensive
against the Liberation Tigers of Tamil Eelam rebels in the north.
The Tigers have been fighting since 1983 to establish a separate homeland for the minority ethnic Tamils.