"The Criminal Investigations Department officers have surrounded the Lanka office and sealed it today," the web edition of the paper said.
A police spokesman said he was not aware of any action taken against the newspaper and the government declined to comment.
The Paris-based Reporters Without Borders urged President Mahinda Rajapakse to end arrests, and what it called intimidation of journalists working for privately owned and foreign media.
"This wave of post-election violence could cast a lasting stain on the start of President Rajapakse’s second term and bodes ill for the political climate during the coming years," the group said.
Reporters Without Borders said a political reporter disappeared two days before January's presidential election while another editor of an an