Ukrainian President Petro Poroshenko reviews an honor guard after the inauguration ceremony in Sophia Square in Kiev, Ukraine, Saturday. Petro Poroshenko took the oath of office as Ukraine’s president, assuming leadership of a country mired in a violent uprising and economic troubles. The Associated Press

Ukrainian honor guards head to the ceremony of hoisting Ukrainian national flag after the inauguration ceremony of Ukraine's new President Petro Poroshenko in Sophia Square in Kiev, Ukraine. The Associated Press

A woman holds a flag of the self-proclaimed Luhansk republic during a rally against the inauguration of Ukraine's new president in Luhansk, Ukraine, Saturday. Ukraine’s new president , Petro Poroshenko, on Saturday called for dialogue with the country’s east, gripped by a violent separatist insurgency, and for armed groups to lay down their weapons but said he won’t talk with rebels he called “gangsters and killers.” The Associated Press

Poroshenko attends the ceremony of hoisting the Ukrainian national flag and Presidential flag after the inauguration. The Associated Press

Ukraine's new President Petro Poroshenko leaves parliament after the inauguration ceremony in Kiev, Ukraine, Saturday. The Associated Press

From left, U.S. Vice President Joe Biden, U.S. Sen. John McCain (R-Ariz.), and Rep. Marcy Kaptur (D-Ohio), attend the inauguration ceremony of Ukraine’s new President Petro Poroshenko. The Associated Press