Ukraine's security service says it is investigating several opposition leaders suspected of attempting to seize power amid the massive anti-government protests gripping the country.

Sunday' statement by Ukraine's Security Service raised the stakes in the opposition's standoff with President Viktor Yanukovych.

The announcement came hours after hundreds of thousands of angry Ukrainians occupied the capital's main square and blockaded approaches to key government buildings during the third straight week of such protests.

The opposition responded by saying that any possible attempts to impose a state of emergency would lead to even more protests.

Sunday's protest — the biggest in Ukraine since its pro-democracy Orange Revolution in 2004 — denounced Yanukovych's recent decision to turn this ex-Soviet republic away way from Europe and closer to Moscow.