MOSCOW (REUTERS, AFP) - President Vladimir Putin said on Wednesday (April 21) that Moscow would respond in a harsh and swift way to any foreign provocations after complaining of continued unfounded and unfriendly acts against Russia.

Speaking at his annual state-of-the-nation address to top officials and lawmakers from both houses of Parliament, Mr Putin said Moscow strived to have good relations with other countries and hoped no foreign state would cross Russia's "red lines".

He said: "I hope that no one will think of crossing a red line with Russia. And we will decide ourselves where it will be."

The longtime Russian leader's annual speech sets the political agenda and last year saw Mr Putin lay out a plan that eventually gave him the power to stay in office until 2036.

This year's address comes as Moscow is locked in diplomatic disputes with the United States and Europe over a Russian troop build-up on Ukraine's borders and a series of espionage scandals.

But the speech was overshadowed by threats of countrywide demonstrations planned in more than 100 cities across Russia on Wednesday evening in support of jailed Kremlin critic Alexei Navalny, who has been on a hunger strike to protest against his lack of medical treatment in prison.

More arrests

On Wednesday morning, police searched the Saint Petersburg offices of Navalny and detained his top aide Lyubov Sobol and spokesman Kira Yarmysh, his allies said.

"Searches were carried out from the very morning," his team in Saint Petersburg said on Twitter. "They have already come for the office's cameraman, several volunteers and activists."

Ms Sobol's lawyer Vladimir Voronin said on Twitter that police had pulled her out of a taxi. "According to her, she was detained by many uniformed officers," he wrote.

He later said she was taken to a police station but was being kept in a police wagon.

Ms Yarmysh retweeted her lawyer saying that she was "just detained at the entrance of her building".

The independent monitor OVD-Info said on Wednesday that police had conducted searches and detained activists in at least 20 cities across the country.

Police have issued warnings against joining Wednesday's rallies in support of Navalny, saying they are illegal gatherings.

More than 10,000 people were detained during demonstrations in support of the Kremlin critic in late January and early February.

The opposition figure was arrested on arrival to Russia in January from Germany, where he had spent months recovering from near fatal nerve toxin poisoning.

He is serving two-and-a-half years in a penal colony for violating parole terms on old fraud charges he says are politically motivated.

The European Union and the US have imposed sanctions on Russia over Navalny's poisoning, and on Monday threatened Moscow with further penalties in the event of his death.

Tension with Ukraine

Mr Putin's speech comes amid a Russian troop build-up on the border with Ukraine, where Kiev's forces have been battling pro-Russia separatists since 2014. The build-up has sparked widespread alarm and warnings from the North Atlantic Treaty Organisation.

Ties with the West have also been hit by a series of spy scandals.

A number of European countries have accused Moscow of increasingly aggressive espionage tactics and expelled Russian diplomats.

Washington last week announced sanctions and the expulsion of 10 Russian diplomats in retaliation for what the White House says is the Kremlin's US election interference, a massive cyber attack and other hostile activity. Russia has vowed to respond in kind.

As tensions peaked last week, US President Joe Biden spoke to Mr Putin by phone, inviting him to hold a summit in a third country. The Kremlin has said it is considering the offer.