MOSCOW — Thousands of Russian nationalists rallied across the country on National Unity Day Monday, in a sign of the growing strength of far-right political forces galvanized by an anti-immigrant agenda.

Hardline nationalists have adopted the holiday, which commemorates the liberation of Moscow from Polish invaders in 1612, as an occasion to hold annual "Russian Marches."

This year's rallies were larger and more numerous than in previous years, in a headache for Russian authorities who worry that rising ethnic tensions pose a threat to public order.

At the largest rally, around 8,000 people assembled in an working-class neighborhood on the outskirts of Moscow, police said. Organizers' requests to hold the rally closer to the city center have repeatedly been denied.

"Moscow has only just woken up, and Russians have only just started to recognize their identity," said Alexander Belov, a nationalist leader and an organizer of the march. "With every day Russian nationalists are gaining more and more support across the country."

Police said they detained around 30 marchers for wearing masks or forbidden Nazi symbols, and for other minor public order offenses. No serious disturbances were reported.

Smaller demonstrations, attracting hundreds or dozens of participants, were held in towns and cities across Russia.

Although nationalist organizations attract the active support of only a small minority of Russians, they tap into widespread public concerns over immigration and disenchantment among Russian youths.

Many ordinary Russians are deeply hostile to immigrants from the largely Muslim regions of Central Asia and the Caucasus, blaming them for problems such as crime and unemployment.

A recent survey by the Levada Center polling agency, taken on the eve of Moscow's mayoral election in September, showed that immigration topped voters' concerns. More than half of respondents said it worried them more than any other problem.

BACK TO THE CZARS

President Vladimir Putin first established National Unity Day in 2005 to replace the Soviet-era commemoration of the Bolshevik revolution.

This year's marches come at a particularly sensitive time, less than a month after thousands of youths rioted in a working-class Moscow suburb, Biryulyovo, following the killing of a young ethnic Russian man.

Police later arrested a citizen from the mostly Muslim country of Azerbaijan for the murder.

Maria, a 15-year-old schoolgirl with dyed red hair, said that she attended Monday's Moscow march — her first — because of the incident.

"After what happened in Biryulovo I couldn't not take part. I want to live in a country where immigrants act like guests, not where they own the place," she said, declining to give her last name.

Many of those marching in Moscow waved black, yellow, and white flags, the old monarchist flag of the Romanov dynasty that has in recent years been adopted as a nationalist symbol.

Others carried religious icons, or pictures of the last Romanov Czar Nicholas I and his family, executed by Bolshevik revolutionaries in 1918.

Many marchers carried banners and placards with slogans like "White Power" and "Russia for the Russians."

"We should stop immigrants from coming into Moscow. Give them land so that they live like monkeys, like the Americans did with the Indians," said demonstrator Alexei Shukin, 49, wearing camouflage fatigues.

As the head of a patchwork state with multiple religions and ethnicities, President Putin he can ill afford any escalation in racial tensions, and he has repeatedly called for racial and religious tolerance.

In a bid to head off the nationalists' rising appeal and mobilize public support behind the government, Russian authorities have at the same time adopted elements of the nationalist agenda, however.

For example, the federal and regional governments have recently cracked down on the use of illegal immigrant labor, notably in construction and outdoor markets.

Critics fear that the response may reinforce negative anti-immigrant stereotypes and fuel ethnic tensions.

But such concerns were little in evidence at Monday's Moscow rally, where participants said authorities were doing too little to clamp down on illegal immigration.

"The only way the current authorities, who make money off illegal immigrants, will listen to us is if we move onto the streets," said demonstrator Shukin.