The eight, all linked to Pakistan's key uranium enrichment facility Kahuta Research Laboratories (KRL), were taken in for "debriefing sessions" at the weekend in the wake of reports from nuclear watchdog the International Atomic Energy Agency (IAEA), officials said.
Three of those held are former military officers.
"Investigations are being conducted...there was some information shared by the government of Iran with us and some information shared by the IAEA, and we decided to conduct in-house investigations," foreign ministry spokesman Masood Khan told a weekly press briefing.
"The investigations are continuing and we haven't made a definite conclusion. Let's not jump to conclusions, there's no presumption of guilt."
Pakistan has received a barrage of accusations over the past year that its scientists passed on nuclear knowledge to Iran, North Korea and Libya in violation of nuclear non-proliferation commitments.
The government firmly rejected the accusations until last month, when it began questioning Abdul Qadeer Khan, credited as the father of Pakistan's nuclear program, and two KRL directors over alleged leaks to Iran.
"Questions have been asked from Mr A.Q. Khan, yes questions were asked from him," the spokesman confirmed Monday.
Masood Khan said in December that individual scientists may have passed on nuclear secrets for "personal greed."
But he stressed Monday no government entity has been involved in nuclear proliferation.
"The government of Pakistan has never proliferated and never will proliferate," he told the briefing.
"No government institution has ever been involved in such transactions or transfers."
Repeating his remarks from December 23, the spokesman said that individual scientists "might have been motivated by personal greed or ambition."
Among the eight taken in over the weekend are retired Major Islam ul-Haq, Qadeer Khan's principal staff officer, a senior official told AFP.
He was arrested as he dined at Qadeer Khan's residence in Islamabad on Saturday night, Islam's wife Nilofer said.
Qadeer Khan, a metallurgist who introduced the designs for uranium centrifuges to Pakistan in the 1970s, headed KRL until retiring in 2002.
Centrifuges are used to manufacture weapons-grade uranium and to create nuclear fissile material.
KRL directors Yasin Chohan and Farooq Mohammad were also taken from their homes in December for questioning. Chohan has since returned home but Farooq is still being held.
Opposition parties lambasted the government for holding the scientists and associated officials, accusing it of betraying people regarded as national assets.
"We strongly condemn the actions being taken against Pakistani nuclear scientists because it is against our national security," said Shahid Shamsi, spokesman for Pakistan's largest Islamic party Jamaat-i-Islami.
"Our scientists are national assets, but the kind of treatment being meted out to them is tantamount to mortgaging them to a foreign country."
The New York Times reported in December that information Iran turned over to the IAEA had strengthened suspicions that Pakistan sold key nuclear secrets to Iran.
The Washington Post reported that Pakistan was suspected of providing centrifuge designs, providing a "tremendous boost" to Iran's pursuit of nuclear weapons.
Pakistan went public as a nuclear power in May 1998 when it conducted underground nuclear tests in response to similar tests by rival India.
The size of its nuclear arsenal is a highly-guarded secret but nuclear research bodies estimate it has 30 to 50 warheads.