(KIEV, Ukraine) — Crowds of pro-Russian demonstrators stormed government buildings Sunday in two major cities in eastern Ukraine, where secessionist sentiment has sparked frequent protests since Ukraine’s Russia-friendly president was ousted in February. In Luhansk, 30 kilometers (20 miles) west of the Russian border, hundreds of people surrounded the local headquarters of the security service and later scaled the facade to plant a Russian flag on the roof. In Donetsk, to the southwest, a large group of people surged into the provincial government building and smashed windows. A gathering of several hundred, many of them waving Russian flags, then listened...