* Opposition parties demand security guarantees
* Say leader Diallo was attacked
* June 30 election date expected to be pushed back (Adds opposition group's decision)
By Saliou Samb
CONAKRY, June 20 (Reuters) - Guinea's opposition parties on Thursday pulled out of U.N.-mediated election talks with the government, accusing police and youths of attacking one of their leaders and his supporters.
The negotiations aim to secure opposition participation in long-delayed parliamentary polls, which are meant to seal the mineral-rich nation's transition to civilian rule following a coup in 2008.
Cellou Dalein Diallo, who heads the UFDG party, announced earlier on Thursday that he was pulling out of the process after the attack by stone-throwing youths and police near his home in the capital Conakry on Wednesday.
The opposition said 17 people were injured in the clashes.
Other opposition parties decided to withdraw from the talks after a meeting on Thursday.
"We decided to suspend our participation in the dialogue to protest against the aggression suffered yesterday by former Prime Minister Diallo," said opposition spokesman Aboubacar Sylla.
He said the opposition would not return until their security was guaranteed.
"Diallo is the opposition's torch-bearer. He was a candidate in the second round of the 2010 presidential election. If he is not spared, nobody is safe," Sylla said.
The government said the violence had been provoked by youths from Diallo's own neighbourhood.
Diallo won the first round of the presidential vote with 43.69 percent against President Alpha Conde's 18.25 percent, but lost in the runoff to Conde a few months later in a ballot marred by violence.
He had been returning from an appearance in court on Wednesday, where he was facing defamation charges brought against him by a Conde ally.
"They can't attack us like this, fire tear gas at us, allow thugs to throw stones at us and expect us to go along with it," Diallo said. "For now, the UFDG is suspending its participation in the dialogue."
More than 50 people have been killed over the past three months during protests by the opposition, which accuses Conde of stuffing the electoral roll with his ethnic Malinke supporters.
Political instability following the military coup has deterred some investors despite Guinea's large deposits of iron ore, bauxite, gold and other minerals.
The elections commission, known as the CENI, said this week that a June 30 date for the polls, rejected by the opposition, would need to be pushed back. (Writing by Joe Bavier and Bate Felix; Editing by Andrew Roche)