the United States and UK have expressed concern after official results from Zimbabwe's elections gave President Robert Mugabe a seventh term in office amid claims of electoral fraud.
US Secretary of State John Kerry stated the results did not "represent a credible expression of the people".
But the regional power South Africa has congratulated Mr Mugabe on his victory.
Mr Tsvangirai, though, rejected the vote for parliament and president as fraudulent and vowed to take legal action.
He stated his Movement for Democratic Change (MDC) would no longer work with Mr Mugabe's Zanu-PF party and boycott government institutions.
The two parties have been in a coalition since 2009, after the last election sparked widespread violence.
Results from this week's parliamentary election handed the MDC a defeat. It won just 49 seats compared with Zanu-PF's 158.
South Africa issued a statement from President Jacob Zuma extending "profound congratulations" to Mugabe following the "successful, harmonised elections"
Major Western groups were not invited to send observer missions to monitor Wednesday's election.
the United States has described the vote as "deeply flawed".
"In light of substantial electoral irregularities reported by domestic and regional observers, the United States does not believe that the results represent a credible expression of the will of the Zimbabwean people," Mr Kerry stated.
Former colonial power the UK also expressed "grave concerns" over reports of large numbers of voters being turned away from polling stations.
British Foreign Minister William Hague urged a thorough investigation of all allegations of violations.
Meanwhile the European Union which maintains sanctions on Mr Mugabe and his senior aides, stated it was worried about "alleged irregularities and reports of incomplete participation" in the election.
Monitoring groups also disagreed over the conduct of the election.