KARACHI, Pakistan — A trash-strewn dusty street here became a front line in recent ethnic battles that killed 100 people in four days.

Now, in the aftermath, residents speak of the street as though it is a chasm, dividing the population of this oceanside city of 18 million and even Pakistan itself.

On one side, people known as Mohajirs, long the dominant group in this economic hub, seethingly point to bullet-scarred and burnt houses and demand a new province that would be theirs alone. On the other side, Pashtuns who migrated here in recent years after fleeing an Islamist insurgency in their native northwest also point to bullet holes, and some express worry that a sort of ethnic cleansing is to come.

“Now they are asking for their own province,” Adnan Khan, a Pashtun whose brother was shot dead by unknown assailants this month, said of the Mohajirs. “Next maybe they will ask for their own country.”

Karachi, Pakistan’s most diverse city, is once more spewing violence that goes unchecked by police and is stoked by thuggish politicians. While the fierce Taliban insurgency seeks to overthrow the government from mountain hideouts hundreds of miles away, the city’s battles are laying bare the deep ethnic, political and sectarian cleavages that pose an additional threat to this fragile federation — as well as an impediment to its unity against Islamist militancy.

When Pakistan parted from India in 1947, it fused vast spans of ethnically and linguistically distinct populations under the common cause of Islam. But the state has struggled to define Islam’s role as a social adhesive. The powerful, Punjabi-dominated military, meanwhile, has aimed to suppress various nationalist movements, even while sometimes backing ethnic and sectarian groups as tools for influence. Politics remain cutthroat and largely localized. The result, some say, is a nation hobbled — and increasingly bloodied — by factionalism.

“Why are they fighting in Karachi? Because they have not become Pakistani yet. People have not become a nation,” said Sayed Jalal Mehmood, the Karachi-based leader of a small nationalist party that represents people native to Sindh province.