By Barbara Plett
BBC News, Islamabad
Many see great dangers in the bitter power struggle
Nawaz Sharif has won the latest round in an ongoing battle with the government, driving in triumph through a police cordon surrounding his house in Lahore.
After a jubilant reception from thousands of supporters, Pakistan's main opposition leader headed to Islamabad.
He is determined to lead lawyers and political activists in the final leg of what is called a Long March to push their demand for the restoration of the deposed chief justice.
It is not clear how far he will get: the government has sealed the capital to prevent protestors from fulfilling their plan to hold an indefinite sit-in in front of parliament.
What is clear is that the challenge has Pakistan's military establishment and international allies very worried.
Top diplomats including US Secretary of State Hillary Clinton have called Mr Sharif and the President, Asif Zardari, in recent days. The army chief of staff has been holding meetings with the prime minister. All are urging a political resolution to the crisis.
It is obvious by now that this is about more than judges.
Of course it is true that the restoration of the chief justice, sacked two years ago by the then military leader Pervez Musharraf, has been a bone of contention between Mr Sharif and Mr Zardari for months.
Mr Sharif accuses the president of failing to honour three written agreements to reinstate the top judge. Mr Zardari argues it is a constitutionally complex issue, but his critics say he is afraid the restored judiciary would challenge aspects of his rule.
However, the controversy over the Long March mushroomed into a political crisis several weeks ago, when the Supreme Court disqualified Nawaz Sharif and his brother Shahbaz from holding elected office.
The Sharifs saw this as a deliberate attempt, backed by the president, to remove them from power, despite Mr Zardari's denials.
That suspicion was reinforced when Asif Zardari imposed federal rule on the Punjab province - the power base of Nawaz Sharif's Muslim League Party (PMLN). In effect the Sharifs' government was dismissed while the president's representatives began negotiations with other parties to permanently replace the PMLN.
Why is this power struggle so dangerous?
First of all, because it puts the main opposition party into a violent confrontation with the government. That harkens back to the instability of the 1990s, when Mr Sharif's Muslim League and Mr Zardari's Pakistan Peoples' Party traded terms in power, each undermining the other.
Secondly, it pits the centre against the Punjab. Again history demonstrates that when the two are in conflict, the government struggles to function.
Some observers believe Mr Zardari may have tried to remove the Sharifs' provincial government for that reason.
The president can mobilise the resources of the state, but Mr Sharif can mobilise popular and material resources, such as his efforts to send thousands of people to lay siege to the capital.
Third, it pits the country's two biggest parties against each other.
The Pakistan Peoples' Party has representation across the country, particularly in the province of Sindh, Mr Zardari's base. Nawaz Sharif's Muslim League is the party of the Punjab, the largest and wealthiest province. Both can rally the street if they want to, violence cannot be ruled out.
Some observers have also expressed concern at rhetoric which appears to endanger the fragile threads of Pakistan's federation, implying that a Punjabi leader is trying to destabilise a Sindhi president.
The West is alarmed because it wants the Pakistani political forces to focus on battling the Taleban and al-Qaeda on the Afghan border, not each other.
The military is alarmed because it sees threats to the integrity of the state. Few believe it wants to intervene. Few doubt it will if the situation descends into chaos.
After Mrs Clinton's intervention, President Zardari has offered to appeal against the Supreme Court decision, but that has not ended Nawaz Sharif's support for the Long March.
A reversal of the court ruling may lead to the reinstatement of his government in the Punjab, but the opposition leader has made it clear that any political resolution of the crisis must include the restoration of the chief justice.