MOSCOW—Russian authorities have restricted access to the website of imprisoned opposition leader Alexei Navalny and to dozens of sites run by his close allies, Navalny’s team said Monday. The action came amid mounting government pressure on opposition supporters, independent journalists, and human rights activists in Russia ahead of the country’s parliamentary election. The September vote is widely seen as an important part of President Vladimir Putin’s efforts to cement his rule before a 2024 presidential election. The 68-year-old Russian leader, who has been in power for more than two decades, pushed through constitutional changes last year that would potentially allow him to hold onto power until 2036. Navalny’s website, as well as the website of his top strategist, Leonid Volkov, and longtime ally Lyubov Sobol were unavailable on Monday. The websites of Navalny’s Foundation for Fighting Corruption and a network of about 40 regional offices, which the Russian government outlawed …