Zardari went to the China Kitchen in the post Clifton neighbourhood on Monday evening, surprising both the management of the restaurant and other diners.
The restaurant is located close to Bilawal House, Zardari's private residence in Karachi.
The President shunned his strict security arrangements and "candidly exchanged greetings" with other diners by waving at them and asking them how they were doing, media reports said.
Ghazala, the owner of China Kitchen, said she "couldn't believe her eyes" when she saw Zardari and his daughter enter the restaurant.
She told The Express Tribune that they arrived without prior notice, dressed casually and were "humble" in their manners.
"He complimented us by saying that he liked the simplicity of the restaurant and enjoyed the food. He wanted to know what inspired me to open China Kitchen. While he was leaving, he said, 'Keep up the good work'," Ghazala said.
Zardari was "very friendly" with the other diners, she said.
After the dinner, Aseefa posted on Twitter: "Just had dinner at China Kitchen, it was great."
Some reports said Zardari paid Rs 10,500 though the bill was just Rs 4,500, with the remainder meant as a tip for the servers.
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