By KARL RITTER
KYIV, Ukraine (AP) -- Russian President Vladimir Putin ordered a partial mobilization of reservists Wednesday, taking a risky and deeply unpopular step that follows humiliating setbacks for his troops nearly seven months after invading Ukraine.
The first such call-up in Russia since World War II heightened tensions with the Western backers of Ukraine, who derided it as an act of weakness and desperation.
The move also sent some Russians scrambling to buy plane tickets out of the country, and hundreds of people were arrested at anti-war demonstrations across the country.
In his seven-minute nationally televised address, Putin also warned the West that he isn't bluffing over using everything at his disposal to protect Russia -- an apparent reference to his nuclear arsenal. He has previously told the West not to back Russia against the wall and has rebuked NATO countries for supplying weapons to Ukraine.
The Kremlin has struggled to replenish its troops in Ukraine; there have even been reports of widespread recruitment in prisons.
The total number of reservists to be called up could be as high as 300,000, officials said. However, Putin's decree authorizing the partial mobilization, which took effect immediately, offered few details, raising suspicions that the draft could be broadened at any moment. Notably, one clause was kept secret.
Despite Russia's harsh laws against criticizing the military and the war, protests erupted across the country. More than 800 Russians were arrested in anti-war demonstrations in 37 cities, including Moscow and St. Petersburg, according to the independent Russian human rights group OVD-Info.
An Associated Press crew in Moscow witnessed at least a dozen arrests in the first 15 minutes of a protest in the capital.
"I'm not afraid. I'm not afraid of anything. The most valuable thing that they can take from us is the life of our children. I won't give them life of my child," said one Muscovite who declined to give her name. Asked whether protesting would help, she said: "It won't help, but it's my civic duty to express my stance. No to war!"
The Vesna opposition movement called for protests, saying: "Thousands of Russian men -- our fathers, brothers and husbands -- will be thrown into the meat grinder of the war. What will they be dying for? What will mothers and children be crying for?"
As protest calls circulated online, the Moscow prosecutor's office warned that organizing or participating in such actions could lead to up to 15 years in prison. Authorities issued similar warnings ahead of other protests recently. Wednesday's were the first nationwide anti-war protests since the fighting began in late February.
Other Russians responded by trying to leave the country, and flights out quickly became booked.
In calling for the mobilization, Putin cited the length of the front line, which he said exceeds 1,000 kilometers (more than 620 miles). He also said Russia is effectively fighting the combined military might of Western countries.
U.S. national security council spokesperson John Kirby said Putin's speech is "definitely a sign that he's struggling, and we know that."
The partial mobilization order came a day after Russian-controlled regions in eastern and southern Ukraine announced plans for referendums on becoming integral parts of Russia -- a move that could eventually allow Moscow to escalate the war. The balloting is all but certain to go Moscow's way. Foreign leaders are already calling the votes illegitimate and nonbinding. Ukrainian President Volodymyr Zelenskyy said they were a "sham" and "noise" to distract the public.
Russian Defense Minister Sergei Shoigu, who also spoke Wednesday on Russian TV, said only those with relevant combat and service experience will be mobilized. He said about 25 million people fit that criteria, but only about 1% of them will be mobilized.
A spokesman for Zelenskyy called the mobilization a "big tragedy" for the Russian people.
Putin's mobilization gambit could backfire by making the war unpopular at home and hurting his own standing. It also concedes Russia's underlying military shortcomings.
A Ukraine counteroffensive this month seized the military initiative from Russia and captured large areas in Ukraine that the Russians once held.
In his address, far shorter than previous speeches on the war, Putin accused the West of engaging in "nuclear blackmail," and he alleged "statements of some high-ranking representatives of the leading NATO states about the possibility of using nuclear weapons of mass destruction against Russia."
"When the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal," Putin said.