On Nov. 11, militants demolished a police department used to detain terror suspects in Karachi, killing 18 people and wounding 130 others. Pakistan's Taliban swiftly claimed responsibility.

It is an example that among the myriad problems confronting Pakistan's president, Asif Ali Zardari, none strikes as close to home as the current instability in Karachi. The country's largest city and commercial capital, with a population of 12 million, has become a nerve center for many of the wider and overlapping threats facing Pakistan, including political/ethnic strife, militancy and criminality.

The bombing, of the city's Criminal Investigation Department, brings some of Karachi's problems sharply into focus. While most of Pakistan bears little resemblance to the teeming port city, Karachi's fragile security situation has significant ramifications for the rest of the country.

Since the start of this year and in particular this past month, Karachi has witnessed a surge in political and ethnic violence, threatening the stability of Zardari's administration. The metropolis is home to significant communities of mohajirs (Urdu-speaking migrants from pre-partition India), Sindhis, Pushtuns and Balochis, among others. However, far from living in communal harmony, these groups are highly segregated, with tensions between well-armed factions often erupting into street violence.

This is not a new phenomenon. Karachi is the main power base of the Muttahida Qaumi Movement (MQM) – primarily an ethnic mohajir party – but is the capital city of Sindh province, a traditional stronghold of the ruling Pakistan People's Party (PPP). The PPP, widely perceived as a Sindhi-dominated party, and the MQM were in open conflict throughout much of the 1990s, during which hundreds of civilians were killed and the city was paralyzed. With the recent influx of Pashtun migrants fleeing floods and military operations in the northwest of the country, this ethnic group is gaining in numbers and influence.

As a result, the Pashtun-dominated Awami National Party (ANP) also has a growing presence in the city. The MQM is fearful of losing control of Karachi to Pashtun nationalist parties, but is equally wary of its historical rival, the PPP.

The struggle for influence between the MQM, PPP and ANP, through land grabs and battles for territory, is at the heart of this year's surge in violence. Far from being a mere local issue, the turmoil could threaten the survival of Pakistan's national government. That is because political maneuverings in the aftermath of the 2008 election resulted in a tenuous alliance between the MQM and PPP. With its razor-thin parliamentary majority, the PPP depends heavily on backbench support from the MQM, which has exerted influence mainly by threatening to quit the coalition, potentially forcing fresh elections.

Political calculations aside, Karachi also presents Zardari with a more lethal set of adversaries. The city, a focal point for Islamic extremist groups of different stripes, is one of the key engines driving the machinery of jihad in Pakistan. Karachi's extensive network of radical mosques and madrassas – a legacy of General Zia ul-Haq's military regime in the 1980s - forms a key component of this machinery.

During the Zia era, madrassas in particular were a major recruiting ground for the anti-Soviet jihad in Afghanistan and the violent anti-Shia agenda at home. Since then, the influence of these mosques and madrassas has only deepened in Karachi, underpinning Sunni militancy in the city today and across the country. Indeed, over the past three years, there has been a renewed surge in anti-Shia violence in Karachi, in particular suicide bombings carried out by Sunni extremist groups such as Lashkar-e-Jhangvi.

Perhaps more importantly for Pakistan's wider security situation, as the International Crisis Group has noted, Karachi's mosques and madrassas provide a critical source of recruitment and networking for militants from across the country and the world. The significance of this infrastructure, according to Jane's, is that "these madrassahs, mosques and camps are the arteries that enable the human flow to and from the tribal areas."

This is apparent in the recent flurry of raids in Karachi by Pakistani authorities, who have arrested a number of al Qaeda, Afghan Taliban and Pakistani Taliban leaders, mainly in Pashtun parts of the city.

The state's inability to clamp down on radical militancy in Karachi stems from an under-equipped and corrupt police force, lack of political will and, in some cases, political patronage of extremist groups. As a result, there is growing evidence that the city's law enforcement officers are becoming soft targets for retaliatory attacks by militants without having the resources to properly repel such assaults.

Taliban fighters have been able to exploit such insecurity. Taliban militants from the northwest come to Karachi not just to seek temporary refuge in the city's large Pashtun and Afghan slums, but for funds and other resources to continue their campaign against Pakistani and ISAF forces. Police attribute a rise in kidnappings for ransom and major robberies in Karachi to a growing nexus between criminality and militancy stemming from the tribal belt.

With much of the Taliban insurgency taking place in Pakistan's northwest, it is tempting to treat this region in isolation. However, the country's security challenges cannot be ring-fenced in North or South Waziristan. Indeed, the dense and interwoven array of competing ethnic/political, sectarian, and criminal threats that beset Karachi highlights the complexity of both Pakistan's democracy and its fight against extremism.

Urmila Venugopalan is a freelance writer and consultant. She can be reached at urmila.venugopalan@gmail.com