Two of the suspects detained on Saturday were Indonesian and 13 Malaysian, state-run television said.
"They were arrested for involvement in activities which are against the interests of Pakistan," the report said. It did not elaborate but the term is usually used for "terrorism"-related activities.
"The relevant governments have been informed," the television report said.
Pakistan Television said the Malaysian and the Indonesian governments had withdrawn permission for the students to stay on in Pakistan.
"After completing necessary investigations and legal formalities, they will be handed over to the concerned countries," it said.
Karachi, a sprawling city of more than 14 million people, has been the scene of several bloody attacks on a variety of domestic and foreign targets in recent years, including those on Western and Christian targets.
Islamist radicals angered by Pakistan's support for the United States-led "war on terror" have been blamed for those attacks.
In May 2002, a bomber killed 11 French naval officials and three Pakistani nationals outside the Sheraton Hotel.
A month later, a car bomb outside the US consulate killed at least 11 people and wounded a further 45.
In the latest violent incident, a bomb exploded in an office building in the commercial heart of Karachi on Friday but caused no casualties. Responsibility for the bombing was not immediately clear.
Pakistan has arrested several senior members of al-Qaida, the group blamed for the 11 September 2001 attacks on the US, as well as several hundred more junior members and supporters.
Fighters from south-east Asia, including some from Indonesia - the world's most populous Muslim country - trained at al-Qaida camps in neighbouring Afghanistan before the US-led invasion of that country in late 2001.