WASHINGTON — The United States on Monday voiced concern after Russian police raided the homes of top protest leaders ahead of a planned mass rally in Moscow against President Vladimir Putin.

"The United States is deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations on June 12," State Department spokeswoman Victoria Nuland told reporters.

Police armed with assault rifles carried out a coordinated sweep of the homes of young Russian politicians, who analysts believe represent the biggest threat to ex-KGB spy Putin's 12-year rule.

Nuland also criticized a new law in Russia that imposes "disproportionate penalties" for violating rules on public demonstrations.

Russian police were calling in opposition leaders for questioning one hour prior to the planned rally time on Tuesday in a move "clearly designed to take them off the streets during the demonstration," she said.

"Taken together, these measures raise serious questions about the arbitrary use of law enforcement to stifle free speech and free assembly," she said.

US criticism over freedom of expression has been a sore point in relations with Russia.

Putin, then prime minister as he prepared to return to the presidency, last year lashed out at US Secretary of State Hillary Clinton when she questioned the fairness of parliamentary polls.

Putin accused Clinton of orchestrating protests critical of him. The State Department dismissed the allegations, saying it supports pro-democracy work around the world but not specific parties.

Clinton issued a statement sending best wishes for Russia's National Day on Tuesday that made no direct mention of the concerns voiced by her spokeswoman.

The top US diplomat welcomed growing trade between the United States and Russia and said the United States was "proud" to support Moscow's entry into the World Trade Organization.

"We look forward to even closer ties with the ratification of agreements to increase the duration of business and tourist visas and to strengthen protections and procedures for intercountry adoptions," she said.