A former Supreme Court judge is to stand against President Pervez Musharraf in Pakistan's October presidential election, supporters say.
Gen Musharraf says he will give up his military role after the election
Wajihuddin Ahmad has to file his nomination by Thursday.
Analysts say Mr Ahmad has little chance of beating Gen Musharraf. The president is chosen by parliament and the provincial assemblies.
The Supreme Court is hearing a number of challenges to whether Gen Musharraf is eligible to stand.
Mr Ahmad's challenge to Gen Musharraf was announced by the president of the Supreme Court bar association, Munir Malik.
He was one of a number of Supreme Court judges who refused to sign an oath of allegiance to a new provisional constitution after Gen Musharraf's military coup of 1999.
The BBC's M Ilyas Khan in Karachi says that, in general, Mr Ahmad's career has been clean and uncontroversial.
Pakistan's lawyers have been in the forefront of the challenges to President Musharraf, ever since his ultimately unsuccessful attempt to sack the current chief justice of the Supreme Court.
Our correspondent says that the lawyers may be hoping that if the court rules Gen Musharraf ineligible to stand, then Mr Ahmad would be a candidate acceptable to most other groups.
Meanwhile, police have clashed with opposition activists as the Supreme Court resumed its hearings.
Anti-Musharraf voices are getting stronger in Pakistan
About two dozen people were detained as they tried to mount a protest outside the court in Islamabad, witnesses said.
Roads leading to Islamabad have been closed and key opposition figures were detained over the weekend.
The Supreme Court was asked to consider a number of petitions challenging Gen Musharraf's right to stand in the poll, which the Election Commission says will take place on 6 October.
So far it has rejected three of the petitions.
The biggest objection among opposition parties is that they say he is ineligible to run as long as he retains his powerful position as army chief.
General Musharraf has offered to resign from the military post, but only after he is voted in for a new five-year term.
Heavy security arrangements were put in place to prevent protesters from getting close to the Supreme Court, which is expected to rule this week.
Roadblocks have been put in place on the main Grand Trunk Road in Attock, and commuters travelling between Punjab and North West Frontier Province are being checked.
Hundreds of activists belonging to the umbrella group of opposition parties, All Parties Democratic Movement, have been detained in recent weeks.
Police said the arrests were due to the "prevailing security threat"
They are from the PML-N party of exiled former PM Nawaz Sharif and hard-line Islamists of Jamiat Ulema-i-Islam.
It is not known how many senior leaders have been held so far - estimates vary from four to 35.
Police officials said the opposition leaders had been taken into temporary custody for the maintenance of public order.
If the Supreme Court rules against Gen Musharraf, he is expected to either stand down or take emergency measures to hold onto power, reports the BBC's Barbara Plett in Islamabad
If it rules in his favour, he will submit his nomination papers, beginning the countdown to the October election.
A coalition that includes Pakistan's main Islamic parties and the PML-N of former Prime Minister Nawaz Sharif says that its deputies will resign from parliament on 29 September, rather than take part in the elections for the president.
Mr Sharif will return to Pakistan after the Muslim festival of Eid, his spokesman says. Eid will end on 13 or 14 October.
Mr Sharif tried to return earlier this month, but was deported back to Saudi Arabia hours after landing at Islamabad.
Another former Prime Minister Benazir Bhutto says she will return on 18 October.