Nemtsov served as a governor and deputy prime minister in the 1990s. He later became an opposition leader and sharp critic of Russian President Vladimir Putin. He was killed on a street in Moscow.
Ukraine's military says the withdrawal of 100-millimeter guns "is the first step" in a process that will be monitored by the Organization for Security and Cooperation.
The foreign ministers of the four signatories to the Minsk cease-fire agreement met in Paris to discuss the fighting in Ukraine. Kiev worries that the separatists' next target will be Mariupol.
In a claim that's meeting with skepticism in Kiev, Russian-backed separatists say they've started to withdraw heavy weapons in eastern Ukraine, as required by a recent cease-fire.
Noting deadly attacks by Russian-backed separatists who have renewed a push near the southern port city of Mariupol, Ukraine says it can't withdraw heavy weapons from the front lines.
The explosion, at the demonstration marking the one-year anniversary of the ouster of President Viktor Yanukovych, occurred in the eastern city of Kharkiv.
President Viktor Poroshenko, speaking on the one-year anniversary of a bloody day of protests leading up to his predecessor's ouster, accuses the Kremlin of organizing snipers who killed dozens.
Battles rage nearly a week after a cease-fire deal. Meanwhile, a British parliamentary committee accuses the EU of a "catastrophic misreading" of Russia's designs on Ukraine.