They were released after a few hours, but detained again Tuesday in another unauthorized demonstration.
Navalny was released early Wednesday, then detained again a few hours later at another opposition gathering.
Both were sentenced for failing to obey police orders.
Separately, the state news agency RIA Novosti cited an unidentified law enforcement official as saying dozens of people were detained on Wednesday afternoon on Manezh Square adjacent to the Kremlin.
According to the report, opposition supporters wearing trademark white ribbons had earlier taken part in a rally by the Communist Party marking the Victory Day holiday, then tried to stage the so-called "people's stroll."
Police could not immediately be reached for further comment.
The wave of massive protests that broke out in Moscow after December's fraud-marred parliamentary elections had gone ahead with little interference from police.
Demonstrators, whose numbers sometimes appeared to exceed 100,000, stuck carefully to approved venues and march routes; police presence at these rallies was extensive but restrained.
However, police have taken swift and sometimes brutal action to break up unauthorized demonstrations, while the opposition appears to be determined to try to maintain its momentum.