KARACHI, PakistanKARACHI, Pakistan (Reuters) - When British colonial rulers hastily left South Asia at Pakistan's painful birth in 1947, the ensuing chaos and violence meant little attention was paid to the architecture they built or influenced in Pakistan's biggest city, Karachi.

More than 70 years later, architectural gems have been torn down and many are either crumbling or under threat from real estate developers in Pakistan's commercial capital which is mushrooming into a mega-city.

The structures, weathered by the salty air, open the door to Karachi's colonial scars, researchers say, pointing out that many of the original owners were among millions of Muslim and Hindu refugees who fled their homes amid communal and religious violence that accompanied the end of British rule in India in 1947 and the creation of Pakistan.

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"Every brick of the heritage building narrates a story of those who left in 1947," said Akthar Baloch, a researcher who has written several books on Karachi's heritage. "They built them with love and affection.

"When people like me feel bad looking at the neglect of these heritage sites, one wonders how the families of the owners must feel if they ever visit Karachi." (Click reut.rs/2F03sEg for a picture package of Karachi's crumbling heritage buildings)

Karachi's population has skyrocketed to nearly 17 million people in 2017 from an estimated 400,000 at independence, and every inch of the city has become a valuable commodity for developers building homes or drafting plans to alter the city's skyline with new skyscrapers.

Jahangir Kothari Parade promenade, once an imposing British heritage site, is now obscured by a maze of overpasses and the shadow of Pakistan's tallest building.

The promenade is part of a handful of buildings, along with the colonial-era Imperial Customs House, which have been restored to their former grandeur, but such projects are rare when the focus is on tearing down old and building new.

Rapid urbanization has ensured large-scale destruction, particularly in the old city areas, where more profitable multi-story residential buildings have sprung up.

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But amid the new concrete, remnants of the colonial legacy can still be seen, often recognizable by their state of neglect.

The Saddar neighborhood of Karachi has perhaps the largest concentration of British architectural history, while in the city's eastern district, the iconic old colonial jail has been declared a heritage site by Sindh province's antiquities department.

So far more than 1,700 premises have been listed as heritage sites by the antiquities department and the process continues.

The Sindh Cultural Heritage Preservation Act, introduced in 1994, has helped provide legal protection for structures of historical significance. But courts are also busy with cases of developers trying to circumvent such protection.

(Editing by Nick Macfie)