Source: Associated Press/Huffington Post | Lynn Berry MOSCOW — Vladimir Putin took the oath of office in a brief but regal Kremlin ceremony on Monday, while on the streets outside thousands of helmeted riot police prevented hundreds of demonstrators from protesting his return to the presidency. Putin, 59, has ruled Russia since 2000, first as president and then during the past four years as prime minister. The new, now six-year term will keep him in power until 2018, with the option of running for a fourth term. “I consider service to the fatherland and our nation to be the meaning…