The antiquities had been illegally dug from the country's restive northwest where Pakistan's army is battling against militants.
The latest raid on a warehouse in the eastern Ibrahim Hyderi neighbourhood unearthed two large boxes stuffed with ancient Gandhara art.
The haul included statues of Buddha, life-sized idols, bronze artefacts, pottery and decorative plaques, Qasim Ali Qasim, director of Sindh province archaeology department, told AFP.
On Friday, the police intercepted a flatbed truck in Karachi and found similar antiquities from the 2,000-year-old civilisation hidden under plastic and wooden items.
Senior police official Latif Siddiqui said the driver and cleaner of the truck who were arrested on Friday gave a lead to the warehouse which helped recover more artefacts.
"We have inventorised the antiquities that we had found from the truck on Friday, which are more than 300," Siddiqui said.
"We have yet to inventorise the antiquities we have seized today, which will make the number even more staggering."
Another senior police official, Shahid Hayat, said, "the artefacts include two large statues which we are so heavy that we are having problems moving them with a lifter."
"We are investigating whether the smuggling of the artifacts is part of an international ring of smugglers," he said.
Qasim said it was one of the biggest seizure of such precious antiquities in the country's history.
Qasim had earlier said he believed the items had been dug up in Taliban-infested northwest Pakistan and brought to Karachi a piece or two at a time, ready for dispatch to Europe overland via Afghanistan and Central Asia.
"The thieves and mafias involved in this business dig in the northwest, which is filled with Gandhara sites with little control by the authorities," Qasim had said.
"They dug up ancient pieces, accumulated them in Karachi and then wanted to send them to Afghanistan in the garb of a NATO vehicle when they saw Pakistan has reopened the route."
A security official said the seizure showed an organised mafia was at work to deplete the country of its rich past.
"They must be very influential and well-organised criminals and we need to launch a thorough search to catch them," he said.
Gandhara was a Buddhist civilisation that flourished around the modern-day city of Peshawar and in parts of eastern Afghanistan.
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