Postcard from... Karachi

Karachi, Pakistan's largest and most volatile city, has been in lockdown this weekend.

Mobile phone networks were suspended from noon on Friday, not returning until Sunday evening. Motorcycles were restricted, and many roads blocked off.

The reason? The religious month of Muharram. The peak of the festival comes on the ninth and tenth days, when Shia Muslims turn out onto the streets in their thousands for processions to commemorate the death of Hussein ibn Ali in the seventh century.

Shias are a target for Sunni extremist groups, so there is a high risk of bomb attacks.

The city was tense; the streets strangely deserted – an unusual sight in this metropolis of 18 million people. Saturday came and went peacefully. In the evening, road restrictions were relaxed enough that leaving the house was an option.

That night, there was an explosion at an apartment in the Defence Housing Authority (DHA), an upmarket area of Karachi. Within minutes, there was a police siege outside the house and rolling coverage on every news channel.

Sitting in a house in DHA, not far away, we watched coverage of the "suspected terrorist" tensely, unable to call and check in with family members because of the mobile phone outage.

It ended up being a storm in a teacup. No explosives were found in the house. The blast was caused by chemicals used in the production of narcotics.

The residents of the house, who had moved in just days before, had fled the scene.

Perhaps the terror alert is creating some terror of its own.