An adverse ruling may put pressure on Mr Musharraf
The Supreme Court of Pakistan has asked former president Pervez Musharraf to explain his decision in November 2007 to impose emergency rule.
The notice was issued after hours of debate in the courtroom over whether it was appropriate to make him a party to a case challenging his actions then.
He imposed emergency rule, suspended the constitution and dismissed about 60 judges after challenges to his power.
Mr Musharraf does not have to appear in person in court and can send a lawyer.
The court took the view that anybody whose actions were being discussed in a case had the right to be represented.
"This is the first time in Pakistani history that the court has taken cognisance of such action. In the past, the courts have tended to condone military takeovers," a former chief justice of Pakistan, Saeeduzzaman Siddiqui, told Dawn News TV.
The BBC's Ilyas Khan in Islamabad says that the court apparently issued the notice to Mr Musharraf following the refusal on Tuesday by the attorney-general, who represents the government, to defend the former president's position in the case.
However Malik Qayyum, a former legal aide of Mr Musharraf, told the media outside the courtroom on Wednesday that he would consider representing him.
Correspondents say the notice comes at a sensitive time in Pakistani politics, as Islamabad is under increasing US pressure to eliminate the Taliban and al-Qaeda along the Afghan border.
Mr Musharraf - who is currently on a lecturing tour abroad - imposed emergency rule on 3 November 2007 when faced with growing challenges to his rule as president and weeks after his controversial re-election for a second term.
The judges who replaced the 60 who were dismissed may also lose their jobs if the court declares the presidential action illegal.
At the time of their appointment, they were asked to take a fresh oath of office under an interim constitutional order issued on the same day.
Petitioners have now challenged that order, and are pleading that judges who took the fresh oath be stopped from sitting on the bench.
This will require the court to determine the legality of the president's action, experts say.
Any decision which upholds the challenge will put pressure on the government to start legal action against Mr Musharraf for alleged high treason, they say.
The Supreme Court has constituted a 14-member bench to hear the case.
The bench comprises judges who were sacked in 2007 for refusing to take the fresh oath.
Most of them were reinstated by the newly elected government after August 2008, when Mr Musharraf resigned as president to avoid impeachment by parliament.