Saturday's attack on Zafar Iqbal in the northern city of Sylhet was just the latest in a series of stabbings of secular or atheist authors and bloggers in Muslim-majority Bangladesh.
Prime Minister Sheikh Hasina, whose government has waged a fierce crackdown on homegrown extremism in recent years, blamed "religious fanatics" for the assault.
Iqbal, a longstanding champion of free speech and secularism, remains in a stable condition after being flown to a military hospital in Dhaka with stab wounds to his head.
Police detained 21-year-old Faizul Hasan, a former Islamic seminary student, and were investigating any ties to the radical groups.
Colonel Ali Haider Azad Ahmed from the elite Rapid Action Battalion unit said Hasan told investigators it was "his duty as a Muslim to resist those who work against Islam".
"He has said Dr Zafar Iqbal was an enemy of Islam," Ahmed told AFP.
Police said Hasan may have had links to extremists blamed for attacks on secular and atheist writers in the last four years.
Suspected Islamist radicals have killed around a dozen such writers and bloggers, including an American atheist blogger of Bangladeshi origin.
Police have blamed homegrown Islamist extremist group Ansarullah Bangla Team - also known as Ansar-al Islam and linked to Al-Qaeda in the Indian Subcontinent - for most of the attacks.
Bangladesh's government has provided security for top secular writers and activists since Islamist extremists named them in several lists of targets.
Iqbal, 64, is a bestselling author and celebrity speaker who regularly appears at campuses nationwide.
The attack was swiftly condemned by protesters in Dhaka and Sylhet, who staged torchlit marches and planned further rallies to demand justice later Sunday.