A Ukrainian journalist with a vocal pro-Russian stance was killed in Kiev, the capital, by unidentified gunmen on Thursday, a day after a pro-Russian lawmaker was killed in a similar attack. The journalist, Oles Buzina, 45, publicly opposed the protests that led to the ouster of President Viktor F. Yanukovych in 2014. The current president, Petro O. Poroshenko, called for a swift investigation and declared that the recent killings were “conscious provocations” intended to “destabilize domestic politics in Ukraine.” President Vladimir V. Putin of Russia said Ukraine’s government was allowing a campaign of political violence against supporters of the previous government.