MOSCOW (AP) — There were “serious problems” in the vote that returned Vladimir Putin to the Russian presidency, the head of the major international election observer mission said Monday, adding fuel to an opposition testing its strength with plans for a massive protest rally.

Putin, now the prime minister, rolled to victory as expected Sunday to return to the Kremlin and keep his hold on power for six more years, but opponents claim the vote was rigged and the outcome was never in question.