MOSCOW—Russian authorities on Monday ordered the offices of imprisoned opposition leader Alexei Navalny to suspend their activities pending a court ruling on whether they should be outlawed as an extremist group. The injunction from the Moscow prosecutor’s office was another step in a sweeping crackdown on Navalny, President Vladimir Putin’s fiercest critic, and his organizations. The prosecutor’s office petitioned a court earlier this month to label Navalny’s Foundation for Fighting Corruption and network of regional offices as extremist groups. Such a label would outlaw their activities and expose members and supporters to lengthy prison terms, according to human rights advocates. It is a major challenge for Navalny’s embattled team, with its leader in prison and dozens of its members under arrest, targeted for raids by law enforcement or facing criminal charges. The prosecutors also asked a Moscow court to restrict the activities of the foundation by banning it from spreading …