MOSCOW—Russia has detained two allies of jailed Kremlin foe Alexei Navalny, a prominent opposition activist said on Tuesday, adding that they could face extremism charges that carry lengthy prison terms. Russian authorities have in recent months cracked down on groups affiliated with Navalny, 45, who is known as President Vladimir Putin’s most vocal critic. He is serving two-and-a-half years in prison for parole violations related to a fraud case he says was politically motivated. In June, a Russian court ruled that Navalny’s anti-corruption foundation was ‘extremist.’ Then, after his allies regrouped around a new network of organizations set up for Russia’s parliamentary election in September, authorities started investigating campaign staffers on the same extremism charges. On Tuesday, state investigators detained the former heads of provincial election headquarters in the Siberian cities of Tomsk and Irkutsk for questioning, Leonid Volkov, a Navalny ally living in exile, wrote on messaging app Telegram. …