MOSCOW - Sharp-tongued and defiant, Prime Minister Vladimir V. Putin denounced those protesting vote fraud as stooges of the West and insisted that Russia's national election was valid. His opponents were undeterred.

In a 41/2-hour marathon call-in show on national TV, Putin aimed to erect a bulwark against a rising wave of discontent. But his disdainful tone appeared likely to only fuel more protests, after a fraud-tainted Dec. 4 parliamentary vote sparked the largest public anger Russia has seen in a generation.

In an appearance lasting from high noon to sunset Thursday in Moscow, a vigorous Putin, 59, defended the election as reflecting "the real balance of power in the country" and rejected calls for it to be rerun. That effectively dismissed opposition allegations that vote fraud had given Putin's United Russia party a majority of seats in parliament.

Putin acknowledged that the tightly controlled political system he crafted during a dozen years in power "may and should move toward liberalization" and proposed that Web cameras be set up in all of Russia's more than 90,000 polling stations ahead of the March 4 election in which he seeks to return to the presidency.

The opposition was not mollified.

"The boorish, disdainful attitude toward the people that Putin demonstrated in today's television show was obvious," said Boris Nemtsov, a prominent opposition leader, in a blog post after the show.

Last week's protests, which included tens of thousands in central Moscow, indicated that Putin's return to the presidential chair he occupied from 2000 to 2008 will not be as easy as had been expected only two weeks ago. United Russia lost about 20 percent of its parliament seats, and critics say that the slim majority it retained was due only to widespread vote fraud.

Opposition groups plan a series of protests in Moscow and St. Petersburg this weekend, hoping to follow up on last week's dramatic demonstrations in more than 60 cities. Officials have allowed up to 10,000 people at each. Another protest planned for Dec. 24 has an attendance limit of 50,000.

In a telling display of anger, the number of people who signed up on Facebook to go to the Dec. 24 rally rose from 18,000 to 21,500 in the hours Putin was speaking on TV.

He also faces a new and charismatic challenger - Mikhail Prokhorov, owner of the NBA's New Jersey Nets. Hours before Putin's TV appearance, the metals and banking billionaire met with supporters.

"I deeply understand the demands and the strivings of the people who took to the streets," Prokhorov said, adding that he may join a followup protest later this month.

In a direct challenge to Putin - though the prime minister's name was not mentioned - Prokhorov announced that his first move if elected would be to pardon jailed tycoon Mikhail Khodorkovsky, who has been in prison since 2003 for tax evasion and fraud - charges widely seen as punishment for defying Putin's power.

Putin has marginalized opposition forces, tightened election rules, and abolished direct elections of governors. He defended the moves as necessary to prevent criminal clans and separatist forces from dominating the gubernatorial elections but suggested he may allow their election in the future.