Chaotic scenes in Lahore yesterday seemed to stand for the plight of all Pakistan. Riot police fired tear-gas; demonstrators waved banners and threw stones. And the leader of the country's opposition Muslim League, Nawaz Sharif, defied what he said was a house arrest order to join the "long march" of protest to Islamabad. The country is now set for the long-awaited, and potentially lethal, duel between Mr Sharif, twice prime minister in the past, and Pakistan's president, Asif Ali Zardari.

Yet again it could be observed, what a difference a year makes. Only 12 months ago, hopes for stability in Pakistan were high. The assassination of Benazir Bhutto and the disorderly election campaign that preceded it had given way not, as widely feared, to violence or a military takeover, but to a serious and largely peaceful election that was judged to be reasonably free and fair. Ms Bhutto's widower, Mr Zardari, and his old adversary briefly made common cause.

Central to their decision was a pledge to reinstate the judges dismissed by Pervez Musharraf in his desperate effort to hold on to power. And for a few months there was an unaccustomed air of optimism, a sense not only that Pakistan had survived the worst, very much against the odds, but that the country's politicians were ready to co-operate for the greater national good and at one in wanting to restore the rule of law.

That this did not happen, and the bitterness of Mr Sharif and many others that it did not, is what precipitated the "long march", currently heading towards the capital. A year, they argued reasonably enough, was quite long enough to wait for the president to honour his promise.

Mr Sharif's – surely calculated, but still courageous – decision to join the protest in person, and the fact that his car was allowed through the police cordon around his residence yesterday, might be said to show President Zardari's strength. More likely, however, it highlights his weakness.

The ambiguity surrounding Mr Sharif's house arrest suggests an inability to enforce it. And without the reinstatement of the judges, the courts, and the forces of law and order are seen in many quarters as lacking legitimacy. The Supreme Court remains the one that Mr Musharraf put in place to support him. It is the same court that disqualified Mr Sharif from politics and barred his brother, Shahbaz, from seeking re-election in Punjab. And it is the same court that recently sanctioned the placing of Punjab, the Muslim League's power base, under direct federal rule.

The opposition's "long march" has, and was intended to have, noble precedents, including Gandhi's peaceful protest of 1930. But mass actions of this kind, however non-violent the intent, risk running out of control. The inadequacies of law enforcement in Pakistan were recently exposed by the attack on the Sri Lankan cricket team, also in Lahore. With the central authorities blocking routes to the capital, it would be foolish to regard a peaceful end to this march as a foregone conclusion.

Somehow, though, the stand-off has to be resolved – preferably without the intervention of Pakistan's weakened military. With the Taliban extending its power in neighbouring Afghanistan, there is now a real danger of instability erupting across the whole region. President Zardari's dilemma is that, if he yields on the judges, the new Supreme Court could in turn challenge his authority. He has to recognise that almost any other course would be worse.