Crowds gathered outside Iceland's parliament demanding the prime minister step down over allegations he concealed investments in an offshore company.
Sigmundur Gunlaughson earlier refused to resign after details about Wintris, which he owned with his wife, were made public in a massive data leak.
He did not declare an interest in Wintris when entering parliament in 2009.
Opposition parties say they plan to hold a confidence vote.
The prime minister says no rules were broken and his wife did not benefit financially.