Russians cast their ballots with muted enthusiasm in national parliament elections Sunday, a vote that opinion polls indicate could water down the strength of the country’s dominant party.

In the far eastern regions along the Pacific Coast where voting began, initial turnout appeared desultory. Four hours after polls opened in the Kamchatka region, 16.5 percent of registered voters had cast ballots.

“It’s very important to come to the polling stations and vote, but many say that it’s useless,” said Artysh Munzuk, a university student casting his ballot in the Pacific port of Vladivostok.

Prime Minister Vladimir Putin’s United Russia party has signaled concern about polls showing it could receive only a bit more than half the votes. It has cracked down on an independent election monitor and warned of political instability.

Only seven parties have been allowed to field candidates for parliament this year, while the most vocal opposition groups have been denied registration and barred from campaigning. Critics say the 7 percent threshold for winning seats is prohibitively high, effectively shutting out most minority views.

Still, the independent Levada Center polling agency released a survey late last month saying United Russia could get only about 53 percent of the vote, well down from its performance in 2007 that gave it an unassailable two-thirds majority in the State Duma, the elected lower house of parliament.

Putin and President Dmitry Medvedev made final appeals for the party Friday, the last day of campaigning, warning that a parliament made up of diverse political camps would be incapable of making decisions.

The view underlines Russian authorities’ continuing discomfort with political pluralism and preference for top-down operation. As president in 2000-2008, Putin’s autocratic leadership style won wide support among Russians exhausted by a decade of post-Soviet uncertainty.

But United Russia has become increasingly disliked, seen as stifling opposition, representing a corrupt bureaucracy and often called “the party of crooks and thieves.” Putin needs the party to do well in the parliamentary election to pave the way for his return to the presidency in a vote now three months away.

With so much at stake, there are doubts about how honestly the election will be conducted. An interim report from an elections-monitoring mission of the Organization for Security and Cooperation in Europe noted that “most parties have expressed a lack of trust in the fairness of the electoral process.”

The websites of Ekho Moskvy, a prominent, independent-minded radio station, and Golos, the country’s only independent election-monitoring group, were down on Sunday. Both claimed the failures were due to denial-of-service hacker attacks.

“The attack on the site on election day is obviously connected to attempts to interfere with publication of information about violations,” Ekho Moskvy editor Alexey Venediktov said in a Twitter post.

Golos has come under strong pressure in the week leading up to the vote.

Golos’ leader, Lilya Shibanova, was held at a Moscow airport for 12 hours upon her Friday return from Poland after refusing to give her laptop computer to security officers, said Golos’ deputy director Grigory Melkonyants. On Friday, the group was fined the equivalent of $1,000 by a Moscow court for violating a law that prohibits publication of election opinion research for five days before a vote.

Putin last Sunday accused Western governments of trying to influence the election. Golos is funded by grants from the United States and Europe.

The group has compiled some 5,300 complaints of election-law violations ahead of the vote. Most are linked to United Russia. Roughly a third of the complainants — mostly government employees and students — say employers and professors are pressuring them to vote for the party.