By M Ilyas Khan BBC News, Karachi

In pictures: Suicide blast

The suicide bombing that killed more than 20 policemen in the Pakistani city of Lahore on Thursday comes a day before the start of Muharram, the first month of the Islamic lunar calendar.

It also comes just a day after the government finalised security arrangements for the holy month, which is often marred by sectarian violence between Shia and Sunni Muslims.

The eastern city of Lahore, where the attack took place, is among 35 districts that the government has declared "sensitive" during the holy month.

The attack was apparently not sectarian in nature - it targeted the policemen who were on security duty near a lawyers' rally.

Ultimate martyrdom

Also, militants have been targeting police and military personnel over the last couple of years.

But the view that the attack may have been meant to set the stage for Muharram-related violence in the coming days cannot be ruled out.

Lahore eyewitnesses speak

The timing and the pattern appear to conform to militant attacks last year that targeted the police guarding Shia mourning ceremonies and processions.

The month of Muharram marks the events leading to the 7th Century martyrdom of the Prophet Muhammad's grandson, Imam Hussain, at the hands of the then Muslim ruler, Yezid bin Muawiyah.

Traditionally, the Shia Muslims observe these days as a period of mourning, with ceremonies depicting the day-by-day build-up to the ultimate martyrdom at Karbala, Iraq.

Heretical

Since the 1980s, these days in Pakistan have also meant attacks on Shia ceremonies and processions by hardline groups within the Sunni Wahhabi sect.

Originally imported from the Middle East, the purist Wahhabi ideology as espoused by some extremist clerics considers Shias as heretics who deserve to die.

The Wahhabis created a strong local following among Afghan and Pakistani mujahideen who fought against Russian troops during the Soviet invasion of Afghanistan.

Over subsequent years, the mujahideen formed different militant groups, fighting anti-Taleban forces in Afghanistan, Indian troops in Kashmir, and Shia Muslims in Pakistan.

During the 1990s, sectarian violence at or around Shia or Sunni religious occasions became the norm, and was suspected to have been used by the country's intelligence apparatus - which is accused of controlling some such groups - to destabilise successive elected governments.

The last high-profile sectarian attacks took place in 2004, after which direct bombings of Shia or Sunni mosques and worshippers by rival militants have become few and far between.

Fears rekindled

Analysts say this happened because Sunni militants increasingly came into conflict with the government.

Although ideologically these groups still consider Shias as heretics, opening multiple fronts by hitting directly at Shias at this stage appeared to them to be a bad strategy.

This became apparent last Muharram, when a series of bombings and rocket attacks created panic in large parts of the country.

The attacks were not directed at Shia mourners, but at the law enforcement agencies. Scores of policemen were killed during the mourning season, including the police chief of the north-western city of Peshawar, and his deputy.

But most of those attacks took place very close to where Shia ceremonies were being held, creating panic among the mourners and leading many Shias to believe the attackers' aim was also to disrespect their ceremonies.

The attack in Lahore, coming as it does on the eve of the start of Muharram, has rekindled those fears.