WASHINGTON DC, United States?The US said Monday it was "concerned" about the treatment of about 100 protesters detained by Russian authorities during a peaceful weekend demonstration in Moscow.

"The United States is concerned by reports that authorities in Moscow on January 31 once again broke up a peaceful demonstration by Russian citizens," the US State Department said in a statement.

"The detention of at least 100 protesters, including prominent human rights defenders and opposition political leaders, together with reports of mistreatment against some of the demonstrators, constitutes another blow against freedom of speech and assembly, which are universal and fundamental rights that deserve to be protected and promoted," the US statement said.

A spokesman for the Russian opposition Solidarnost movement said the detainees have since been released.

Earlier Monday, the European Union urged the Russian government to stop its "heavy-handed" treatment of protesters, after riot police arrested the opposition activists at Sunday's rally in Moscow.

The EU said the detained protesters included Oleg Orlov, who last year won the European Parliament's Sakharov human rights prize.

Activists in Moscow have been holding regular protests?routinely denied authorization and dispersed by riot police?in defense of the constitutional right to gather in public.