Turkey’s referendum process was held on an “unlevel playing field”, with campaigning restrictions and late procedural changes, monitors say.
Opposition events were obstructed and state resources misused, the Organization for Security and Co-operation in Europe (OSCE) said.
In the referendum, voters gave sweeping new powers to President Erdogan.
The narrow vote was ruled valid by Turkey’s electoral body, despite claims of irregularities by the opposition.
The OSCE said in a statement: “[The] referendum took place on an unlevel playing field and the two sides of the campaign did not have equal opportunities.
“The campaign rhetoric was tarnished by some senior officials equating ‘No’ supporters with terrorist sympathisers, and in numerous cases ‘No’ supporters faced police interventions and violent scuffles at their events.”
President Recep Tayyip Erdogan’s push for an executive presidency succeeded with 51.4% voting for it.