Egypt's biggest opposition movement, the Muslim Brotherhood, is to stand in a third of the seats in next month's parliamentary elections.
The Islamist group, which is banned, won 20% of the seats in parliament in 2005 by fielding candidates as independents.
Leading opposition figure Mohammed ElBaradei had earlier called for a boycott of the vote.
But the group said it would contest 169 of more than 500 seats available.
The Muslim Brotherhood's success in 2005 shocked the political establishment and came despite reports of vote rigging.
It has since faced a heavy government crackdown and some of its senior figures have been detained.
Its leader Mohammed Badie said he wanted to encourage civic duty and confront unfairness.
"We ask all Egyptians to stand firm against any attempt to rig the elections and we call on the government to ensure a fair election," he told reporters on Saturday.
Mr Badie added that an unfair vote would cast a shadow over the 2011 presidential elections.
President Hosni Mubarak, 82, has ruled Egypt for 30 years but has not yet said whether or not he will stand for a sixth term.
Mr ElBaradei, a former head of the International Atomic Energy Agency who returned to Egypt in February, has worked closely with the Muslim Brotherhood to to petition for electoral reform.
He has argued that boycotting parliamentary elections would deny the government legitimacy.
But a Muslim Brotherhood spokesman said that standing would provide a chance to promote alternative ideas.