By Maria Kiselyova MOSCOW (Reuters) – A dispute between shareholders in Russias biggest social network escalated on Monday, as the rivals took legal action to assert their influence over a business that has been used as a platform for opposition groups. Known as Russias answer to Facebook, VKontakte has been the focus of an ownership battle since last April when the United Capital Partners (UCP) fund bought 48 percent of the shares, becoming its then biggest shareholder. Rival Mail.Ru, which has in the meantime joined forces with VK founder and chief executive Pavel Durov to become the biggest investor, contested the UCP transaction in a lawsuit filed by a subsidiary. Durov has clashed with authorities for providing a forum for opposition activists to organize protests against Vladimir Putin, including in 2011 when tens of thousands came out on to the streets in the biggest challenge yet to the Russian leader.