Reacting to a US court sentencing a Pakistani woman scientist to 86 years in jail, protestors chanted "Death to America" and burned the effigy of President Barack Obama.
A New York court found Dr Aaafia Siddiqui, who is said to have links with Al Qaeda, guilty on several counts of terrorism.
The Pakistan government has said it would petition Washington to secure the repatriation of the mother of three on humanitarian grounds.
The police had to fire tear gas shells to prevent activists from marching onto the US consulate in Karachi, which is the hometown of the jailed scientist, who was arrested by the US from Afghanistan.
Police official Javed Akbar Qazi said at least 14 people were arrested for creating a disturbance.
The scientist's sister, Fauzia Siddiqui said the sentence smacked of US bias against Muslims and blamed the Pakistan government for failing to secure Aafiaa's release.