The billionaire philanthropist George Soros got a wet reception in Ukraine when two young men threw water and mayonnaise at him.
Soros was undeterred by the incident and opened the conference
The incident happened at a human rights conference in the capital, Kiev.
The two men were immediately arrested by the police. The radical nationalist group Bratstvo (Brotherhood) claimed responsibility for the attack.
The group accused Mr Soros of trying to repeat in Ukraine the Georgia-type "velvet revolution" of last November.
Some critics have suggested that Mr Soros' Open Society Institute played a role in the ouster of Georgian President Eduard Shevardnadze, who was forced resigned amid mass public protests.
But Mr Soros denied such claims.
"Everything in Georgia was done by its people, not by me. I had nothing to do with it," he told reporters later on Wednesday.
He also dismissed media speculation that the presidential administration was somehow involved, describing the accident as an "individual action".
The Hungarian-born billionaire has founded the Open Society Institute to promote civil society in the former communist countries in Central and Eastern Europe.
Mr Soros is currently on a five-day visit in Ukraine.