Prague - Greenpeace activists Wednesday briefly flew an EU flag at the Czech president's official Prague Castle residence, where current occupant President Vaclav Klaus has been refusing to do so.

An activist partially climbed a flagpole and unfurled the flag for about 10 minutes shortly before Klaus, an outspoken EU critic, hosted the bloc's executive, the European Commission.

After the meeting Klaus, who opposes deeper European integration and the EU's reform Lisbon Treaty, quipped to reporters that he hoped to "convert" commission President Jose Manuel Barroso on the pact.

The Czech president called for more debate on the charter, while Barroso countered that member states which had signed the accord should also ratify it.

The EU executive's visit ceremonially began the Czech Republic's EU presidency lasting through June 30.

The commissioners' meeting with the Czech government was dominated by talks on the Russian-Ukrainian gas row. The event peaked with a presidency opening gala at the National Theater.

Klaus had excused himself from the festivities, his office confirmed - but declined to say why. (dpa)