"Conditions (for the campaign) were clearly skewed in favour of... Vladimir Putin" while the vote count was "assessed negatively in almost one-third of polling stations observed due to procedural irregularities," international observers led by the OSCE said Monday.
Heidi Tagliavini, the head of the Election Observation Mission of the OSCE Office for Democratic Institutions and Human Rights (ODIHR), called for an investigation into all allegations of electoral violations.
"Although the authorities made some effort to improve transparency, there remained widespread mistrust in the integrity of the election process," the diplomat said in a statement. "As a first step, all allegations of electoral violations need to be thoroughly investigated."
Putin secured almost 64 percent of the vote in Sunday's election, winning back the Russian presidency which he held for two terms from 2000-2008 before his four-year stint as prime minister. His nearest rival, the Communist Party boss Gennady Zyuganov, trailed well behind in a landslide victory.
"The point of elections is that the outcome should be uncertain. This was not the case in Russia," said Tonino Picula, head of the delegation of the OSCE Parliamentary Assembly.
Tiny Kox, the head of the delegation of Council of Europe's Parliamentary Assembly, added: "The competition lacked fairness and an impartial referee was missing."