MOSCOW - President Vladimir Putin signed a bill Sunday to end the election of governors by popular vote, while more than 1,000 opposition activists converged on Constitution Day to denounce what they called his increasingly authoritarian rule. Putin notably chose Sunday to sign one of two measures that critics say could violate Russia's constitution, which was adopted in 1993 under his predecessor, Boris Yeltsin, and was considered one of the main democratic achievements of Yeltsin's rule. The Federation Council, or upper house of parliament, gave the measure the final stamp of legislative approval on Wednesday with a vote of 145-1....