MOSCOW (AP) — Russian President Vladimir Putin on Monday tacitly criticized a popular opposition leader whose efforts to expose official corruption fueled nationwide protests last month.

Putin didn't name Alexei Navalny, but the reference was clear. The president said that it was important to differentiate between those who truly want to fight corruption to "strengthen the state" and those who try to use the fight against corruption to further their own political interests.

A month ago, anti-corruption protests organized by Navalny were the largest and most widespread street demonstrations in Russia in years. He has announced plans to run for president in 2018, when Putin is widely expected to seek a fourth term.

As an example of an anti-corruption fight serving political purposes, Putin pointed to neighboring Ukraine, where protesters calling for an end to corruption ousted the Russia-friendly president in 2014. Putin claimed corruption in Ukraine has worsened under the new Western-aligned government.

Transparency International's annual index of corruption perception, however, has shown a slight improvement in Ukraine, which moved up to 131st in the world in 2016 from 144th in 2013.

Navalny has published numerous investigations about official corruption in Russia. His latest was an hour-long documentary video about Prime Minister Dmitry Medvedev's alleged secret wealth, which received more than 15 million views. Medvedev has refused to comment on the allegations.

Speaking Monday during a meeting with the leaders of Russia's parliament, Putin said doing more to fight corruption would strengthen the Russian state.