In an election that has been intensely watched as a marker for the future direction of Russian politics, an outsider running for mayor of Yaroslavl appeared to be headed for a landslide win Sunday night over the candidate backed by the ruling United Russia party. A victory would give the opposition here a huge, national lift, just three weeks after the election of Vladimir Putin to the presidency demonstrated the continuing durability of the system he has constructed over the past 12 years. And it presents the authorities — in Yaroslavl, a city of 600,000 about 160 miles northeast of Moscow, and in the Kremlin, as well — an unmistakable reminder that politics in Russia has become considerably more challenging since street protests broke out in September. Late Sunday, Yevgeny Urlashov, 44, who ran against corruption and official arrogance, had 67 to 69 percent of the vote, with at least 60 percent counted. His opponent, Yakov Yakushev, the owner of a paint factory, trailed with about 29 percent — virtually the same percentage he polled in the first round of the elections on March 4. Urlashov got 40 percent in that round; two other candidates split the rest.