Fans are filing into the 100,000-seat National Stadium in Warsaw, as Poland and Greece prepare to kick off Friday in the Euro 2012 football championship series.

Poland and Ukraine are co-hosting the 16-nation championship, which will be decided on July 1 in Kyiv.

Russia and the Czech Republic will square off in the other match on Friday.

Prominent political leaders in some European Union member states had threatened to boycott the event due to Ukraine's treatment of former prime minister and opposition leader Yulia Tymoshenko.

Ms. Tymoshenko is serving a seven-year jail sentence after being convicted of abuse of office in a 2009 gas deal with Russia. She is now standing trial on tax evasion charges, which could extend her prison time to 12 years.

The former prime minister denies the charges and says they are part of a campaign by Ukraine's president, Viktor Yanukovych, to remove his strongest political rival.

Western nations say the charges are politically motivated.

Ms. Tymoshenko recently went on a nearly three-week hunger strike after saying she had been beaten by prison guards.

She had experienced chronic back pain, but Ukraine's government has refused to let her travel to Germany for medical treatment.