Russian opposition pledges more protests after 1,000 arrested Police arrested hundreds of protesters for the second week running in Moscow

MOSCOW -- Russia's anti-Kremlin opposition has pledged to hold more protests a day after police arrested hundreds of people for a second weekend running at a demonstration in Moscow calling for fair elections.

Opposition leaders said another protest planned in Moscow for next Saturday would go ahead, and Leonid Volkov, a key lieutenant of Russia's most prominent opposition leader, Alexey Navalny, said they would try to expand them nationwide.

Volkov, on Twitter Sunday wrote that the opposition would "obviously" demonstrate again next Saturday, but said they would not negotiate on its location in Moscow with authorities. The city's mayor's office has offered the protesters a less central location to demonstrate but they have rejected it as an illegal restriction on their constitutional right to peaceful assemble.

"There is no point in trying to agree with people who are arresting the negotiators, using state terror, inventing criminal cases, setting brutal sadists with truncheons on citizens," Volkov said, referring to the city authorities. Other opposition figures have said they are negotiating on location.

As they had a week before, on Saturday, hundreds of heavily-armored riot police cracked down on demonstrators as several thousand of them tried to gather on a central Moscow boulevard.

Although the protests were peaceful, police cleared them forcefully, advancing aggressively in lines with batons drawn and dragging away hundreds of people, often almost seeming at random. Videos posted on social media showed groups of police clubbing protesters on the ground.

The monitoring group OVD-Info said 1,001 people had been detained. Russia's interior ministry put it at around 600 arrests from a crowd of about 1,500, but police are known to deliberately underestimate protest numbers.

The arrests on Saturday were so indiscriminate even some Kremlin supporters were swept up by police. An MP from the pro-government Liberal Democratic Party, Valery Bulygin was detained while praising the police response to a camera crew from the liberal channel, TV Rain. The police were behaving "quite softly," Bulygin said, moments before he was hauled away.

The arrests in Moscow were criticized by Western countries. Germany's foreign ministry called on Russia to release the demonstrators, saying "the arrests are not in proportion to the peaceful character of the protests."

"Yesterday's response undermines the rights of citizens to participate fully in the democratic process," the U.S. embassy in Moscow's spokeswoman wrote on Twitter.

The protests have been sparked by authorities refusal to allow opposition candidates to take part in Moscow's city council elections this September. The issue though has taken on larger significance, seen by Moscow's liberal society as a sign of the Kremlin's intolerance of even low-level political opposition.

Authorities have responded by mounting a broad crackdown, arresting 1,400 people at the previous Saturday's protest. Virtually all of the opposition's top leaders have been arrested, most just before that protest. The majority have been given 2-4 week-jail sentences for organizing unauthorized demonstrations. Last week, Navalny was briefly hospitalized by what his doctor said she suspected was poisoning.

Lyubov Sobol, another key Navalny ally, who has become a face of the protests, told ABC News on Friday that she believed authorities were now determined to smother the protests with force.

"I think the authorities now are trying to find a way out of the crisis. They looked at different options and they went with the toughest option, the way of night-time searches and arrests of opposition leaders," Sobol said, adding she thought there was no "no chance" the opposition candidates would be allowed to take part in the elections without mass demonstrations.

Sobol, one of the opposition candidates kept off the ballot, had been the last remaining protest leader still at liberty before she was detained on Saturday morning. She has been on hunger strike for 22 days. When ABC News reporters met her on Friday at her headquarters -- where staff were nervously keeping watch for police -- Sobol was so weak she struggled to stand.

The current protests are notable for their size and staying power. Some observers have interpreted as a hardening of attitudes among the part of the population opposed to the rule of president Vladimir Putin and that arrests and fines are no longer deterring people as they used to.

"Our country has changed, Muscovites have changed and they want a change. They want political representatives in elections," Sobol said on Friday.

She said authorities had blocked the opposition candidates because they feared it would undermine the "propaganda myth" that the opposition has only 2% of the population's support.

Polls show Putin himself remains popular, but the Kremlin is struggling to deal with the drastic unpopularity of his ruling party, United Russia, which has suffered some surprise election defeats in the past year.

Authorities now seem to be turning to harsher tactics. Police this week charged half a dozen people with organizing and participating in "mass disorders," a serious charge that potentially carries years in jail. Russia's Investigative Committee on Saturday also announced it was planning to open a criminal case against Navalny's anti-corruption foundation, which has exposed alleged graft among senior officials.

It recalls how authorities acted following one of the last major protests in Russia in 2012 ahead of Putin's inauguration as president for a third term. In the so-called "Bolotnaya Affair," named after the square where the protest happened, authorities jailed a dozen people to years in prison in hearings that international rights groups condemned as show trials.

One of those charged this time is Sobol's assistant, Alexey Minyailo, something Sobol said was absurd given she says Minyailo was in court with her during the protest.

But Sobol said she believed the harsh trials would not have the same effect as in 2012.

"When they arrested rank and file activists, that frightened people," Sobol said. "Now society has overcome that pain, it has learnt to resist and there is already not that fear."