Officials say one of those taken into custody in Sunday's raid is a wanted bomb expert.
Information Minister Sheikh Rashid has confirmed that seven people have been arrested and that they are foreigners. Another official said more arrests are expected.
Rashid said the arrests were made by Pakistani security agencies and the detainees included two women and five men.
"We have recovered some weapons, including hand grenades and pistols, and some documents from them," the minister said.
Information from abroad
He said the raid followed information from abroad about the suspects' presence in Pakistan.
"We were looking for one of the foreigners wanted (for) terrorist activity outside Pakistan. In that search the security agencies raided an apartment in Gulistan-e-Jauhar and arrested seven foreigners.
"None of them was wanted in any terrorist activity in Pakistan," Rashid said.
"I cannot tell you about their identities as these are sensitive matters."
Security agencies are investigating whether those arrested were planning an attack in the country.
He said some of those arrested were important people but they were not on the US list of most wanted al-Qaida members.
An intelligence official in Karachi told AFP that two of them were Yemenis, and three, including a woman, were Afghans. The official also said an Egyptian couple was among the seven.
Information about one of the suspects came from the US Federal Bureau of Investigation, the official said, requesting anonymity.
He identified the man wanted abroad as Fazal Mohammad Abdullah al-Misri, who is considered an explosives expert capable of making high-powered bombs.
"He could be handed over to his home country," he said without saying what that country was.
None of the seven was implicated in last month's attempt's to assassinate President Pervez Musharraf in the nearby city of Rawalpindi, nor were they wanted in connection with the recent blasts in Karachi, the officer said.
He said maps of sensitive installations in Pakistan were seized from them.
They are being shifted to Islamabad, he said.
"We are expecting some more arrests," the officer said.