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NIZHNY NOVGOROD, Russia, March 24 (Reuters) - Russian police blocked a march planned by up to 2,000 critics of President Vladimir Putin on Saturday, the second crackdown this month on a public display of opposition to the Kremlin.

Witnesses said up to 100 protesters were arrested and more than a thousand others prevented from reaching a square in the centre of Nizhny Novgorod, one of Russia's five biggest cities, as they gathered for a rally outlawed by authorities.

A police helicopter hovered over Gorky Square and adjacent roads were blocked by lines of riot police, several witnesses told Reuters.

City buses were used to ferry away those arrested and scuffles broke out as police wrestled with chanting protesters and removed flags.

"It wasn't a demonstration of power. It was a demonstration of fear," said Oksana Chelysheva, deputy chairman of the Society of Russian-Chechen Friendship.

The so-called march of the discontented was organised by opposition coalition "Other Russia" and follows a March 3 rally in St Petersburg that blocked the city's main thoroughfare.

Nizhny Novgorod, 400 km (250 miles) east of Moscow, was chosen by opposition leaders keen to spread their message to large urban centres beyond Moscow and St Petersburg. More marches are planned in Russia's two main cities in mid-April.

The anti-Kremlin opposition is in a minority in Russia, where Putin enjoys the support of most Russian voters. Protest leaders are resisting what they call the Kremlin's tightening grip on power and demand a fair presidential election next year.

The constitution requires Putin to step down in 2008. Most observers expect him to back a Kremlin insider to succeed him.

ARRESTS

Alexander Gorbatov, police spokesman in Nizhny Novgorod, confirmed there had been arrests. He did not say how many people had been detained or the reason for their arrest.

Interfax news agency quoted him as saying about 30 people had been arrested.

Three witnesses said between 50 and 100 protesters had been detained. A small number had reached the square -- where organisers had hoped to see between 1,500 and 2,000 -- but were dragged away by riot police.

"Thousands were prevented from assembling in or reaching the square. People were stopped in nearby streets. Even taxis were detained," Chelysheva said.

Many leading opposition figures present at the St Petersburg rally, including former world chess champion Garry Kasparov, did not travel to Nizhny Novgorod.

Kasparov's assistant, Marina Litvinovich, told Ekho Moskvy radio station she had been detained. A Reuters photographer was among several journalists who were also held briefly.

Mikhail, a protester from St Petersburg who gave only his first name, said police at the railway station were checking the documents of everybody who arrived in Nizhny Novgorod by train.

"All those from out of town were detained for the period of the march," he said. (Additional reporting by Denis Pinchuk in St Petersburg)

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