That year Alexander I died having left no direct heir to the throne. According to the law, his brother Constantine should have become emperor, but he abdicated in favor of his younger brother Nicholas I who accepted the throne.
But it was the time when a revolutionary movement sprang up in the upper layers of the society. High rank officers and liberal aristocrats inspired by the ideas of the French Revolution and being against serfdom in Russia, wanted to establish a republic and abolish serfdom. So they refused to swear allegiance to the new emperor and arranged an uprising in front of the Senate building. It took place in December of 1825 and the people who took part in it were called the Decembrists. Among these people there were outstanding personalities who were the friends of Alexander Pushkin. the famous poet.
The Decembrist Uprising was severely suppressed but it greatly influenced the minds of the Russian people and gave a push to further development of revolutionary movements in Russia. This uprising took place in the Senate Square, and in 1925 to the centenary of the event it was renamed to the Decembrist Square.
taken from http://stpetersburg-guide.com, thanks guys!)***