Egypt's Muslim Brotherhood says it won at least 13 seats in the second round of parliamentary elections on Sunday.
Government supporters are accused of widespread intimidation
The opposition group's gains have yet to be confirmed by official results after a day's voting marred by clashes.
The Brotherhood more than doubled its number of seats in the first round of balloting, gaining 34 seats.
The group is banned in Egypt but fields candidates as independents. An Islamist supporter was killed by a gang of thugs in Alexandria on Sunday, monitors said.
It was the first death in an election that has been marred by minor violence and, civil society groups say, widespread intimidation and voting irregularities.
The group claims some of its followers were attacked by supporters of President Hosni Mubarak's governing National Democratic Party on Sunday, while others were prevented from voting by police.
The NDP is expected to maintain its overwhelming majority in parliament.
A Brotherhood spokesman said Sunday's 13 victories were achieved outright, without the need for runoffs.
"The success recorded by the Muslim Brotherhood during the first phase sparked fear in the regime, which cannot bear the presence of opposition in parliament," said group's deputy leader Mohammed Habib in an interview with AFP.
Islamist supporters say many were stopped from voting
"The NDP could see it was going to lose and resorted to violence and thugs against the Muslim Brotherhood. All this was aimed at preventing people from voting," he added.
The interior ministry said most of the violence was started by supporters of Islamist candidates.
"The electoral process has been marred by serious and widespread violations that have undermined the credibility and the integrity of the election," the main civil society monitoring group said.
Egypt is electing 454 members of parliament in three stages. The last round is scheduled to be held on 1 December.