A day of nationwide demonstrations against Vladimir Putin and alleged election fraud began in Russia's far east on Saturday, in a test of the opposition's ability to put pressure on the man who has dominated the country for more than a decade.

Witnesses said about 1,000 people protested in Vladivostok on the Pacific coast and RIA news agency said about 20 were detained in Khabarovsk, a city with almost 580,000 people about 30 km (19 miles) from the border with China.

Thousands of people are expected to turn up for rallies in dozens of cities, from Vladivostok to Kaliningrad nearly 7,400 km (4,600 miles) away to the west, in the biggest political protests since Putin came to power in 2000.

A parliamentary election last Sunday, in which Putin's United Russia party won only a slim majority in the State Duma lower house, showed growing discontent with his rule.

Protesters, who this week staged the biggest opposition rally in Moscow for years, say only widespread falsifications prevented the result for United Russia being much worse.

The ruling party's leaders have denied cheating and Prime Minister Putin has accused the United States of encouraging and financing the protesters.

RIA said about 50 people gathered outside Khabarovsk regional government building, and some had their mouths sealed with tape. Police asked the protesters to leave and, when they refused, moved in to arrest them.

In Vladivostok, where Putin's party was beaten by the communists, police looked on as protesters called for the election results to be annulled and detained activists freed.

Protesters held slogans which read: We are against mass falsifications and The rats should go. Many signed up to become election observers in a presidential election in March which is expected to bring Putin back to the Kremlin.

Police were out in force in Moscow, where tens of thousands of people have pledged on social media to join protests. Many celebrities and prominent journalists have said they will take part.

Dozens of trucks with riot police lined Moscow streets in the two central areas where the rallies were expected to take place. Police vans with flashing lights sped through central Moscow.

Putin and President Dmitry Medvedev have both said that Russians have a right to protest but only within the bounds of permission granted by local authorities who normally allow demonstrations only at specific locations and limit turnout.

The Moscow protest was initially planned for Revolution Square, steps from the Kremlin, where authorities granted permission for only 300 people to protest -- a likely recipe for a crackdown if thousands show up.

City authorities said on Friday they had granted permission for 30,000 people to gather at a less prominent, and less symbolic, location across the Moscow river from the Kremlin.

Putin, who remains Russia's most popular leader in opinion polls, may hope to show his government will tolerate peaceful protests within the limits it sets but will crack down on anyone who ventures outside those boundaries.

Some Putin foes vowed to protest in Revolution Square as planned.

(Writing by Gleb Bryanski, editing by Timothy Heritage)