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Hussein Malla, Associated Press
Hezbollah supporters shout slogans and wave Hezbollah flags, during a rally commemorating "Liberation Day" which marks the withdrawal of the Israeli army from southern Lebanon in 2000, in the southern border town of Bint Jbeil, Lebanon, Sunday May 25, 2014. Hezbollah's leader Sheik Hassan Nasrallah is warning that hard-line foreign fighters in Syria pose a global threat as they return home. Nasrallah accused European countries of easing the flight of extremist fighters into Syria, where they are fighting against the rule of President Bashar Assad.

BEIRUT — The leader of Lebanese Shiite group Hezbollah warned Sunday that hard-line foreign fighters in Syria posed a global threat as they trickled home, accusing European countries of facilitating their joining the rebellion against President Bashar Assad.

Sheik Hassan Nasrallah said European countries didn't think the fighters, thousands of whom have poured into Syria mostly in al-Qaida inspired groups, would return home. Most are from the region, but smaller numbers hold European citizenship.

"Many of them are alive, and some of them have begun returning to their first fronts — in Europe, and other places. This is a threat to the security of Europe, international security, how will these countries deal with them?" he said, speaking via video link at a festival celebrating Israel's withdrawal from Lebanon in 2000.

The Iranian-backed Hezbollah has sent fighters to support Assad's troops. They were instrumental in helping Assad's forces dislodge opposition fighters from their strongholds along the countries' border, severing important supply lines and ultimately weakening chaotic rebels.

The charismatic leader also praised Syria's upcoming presidential elections, expected on June 3, and accused rebels of trying to disrupt the vote that Assad is widely expected to win.

Syria's divided opposition has condemned the vote as a sham as the country enters its year of a conflict that has killed over 162,000 people.

The conflict began with largely peace protests in March 2011, but evolved into a civil war with sectarian overtones. Islamic extremists, including foreign fighters and Syrian rebels who have taken up hard-line al-Qaida-style ideologies, have played an increasingly prominent role among fighters.

As Nasrallah prepared to speak, Shiite neighborhoods in Beirut erupted in celebratory gunfire. Tens of thousands of people turned out waving the bright yellow Hezbollah flag in the southern town of Bint Jbeil, where Nasrallah spoke on screens from an undisclosed location.

Also Sunday, Lebanese security forces arrested radical Muslim cleric Sheik Omar Bakri Mohammed, the state news agency reported.

The National News Agency did not say why Mohammed was arrested in the eastern town of Aley. The Lebanese newspaper The Daily Star said the arrest was over his suspected involvement in clashes in the northern city of Tripoli that killed dozens of people over the past year.

Mohammed returned to Lebanon in 2005, after living 20 years in Britain where he headed a radical Islamic group. Britain has banned him from returning.

It wasn't immediately possible to contact Mohammed's lawyer.

Mohammed was previously sentenced to life in prison in Lebanon in November 2010, following a terrorism conviction, but was released on bail after witnesses recanted their testimony.