The crackdown follows a spate of killings, including atheist bloggers, foreign aid workers and religious minorities in Bangladesh over the past two years.
Security forces in Bangladesh have arrested 11,307 people, including over 100 suspected Islamists, in a nationwide crackdown following a spate of killings of secularists and minorities.
"As many as 3,115 more people were arrested on the fourth day of the drive," Xinhua news agency quoted a senior police official as saying.
Police earlier detained 8,192 people, including 119 suspected militants, in the first 72 hours of the clampdown.
At least 18 people, including atheist bloggers, foreign aid workers and religious minorities, have died in attacks in Bangladesh over the past two years.
Two Hindus were killed in separate incidents last week.
The attacks have alarmed the international community and raised questions whether the government can protect minorities and secular intellectuals in the Muslim-majority country.
Most of the 145 suspected militants arrested so far are members of banned Jama'atul Mujahideen Bangladesh (JMB).
Amid a new surge in militancy, Bangladesh on Friday launched a special drive aimed at dismantling terror outfits.
The crackdown began four days after the wife of a police superintendent, who led operations against Islamist militants and drug cartels, was shot and stabbed to death in Chittagong city.
The killing caused a furore among Bangladesh’s political establishment.
JMB, campaigning for establishment of Islamic rule in Bangladesh, carried out a series of bombings in the country, including Dhaka, on August 17, 2005, leaving two people dead and 150 others injured.
Hundreds of JMB leaders and activists were rounded up while six top leaders of the group, including Shaikh Abdur Rahman, were hanged in 2007.
A number of secularist writers, bloggers and publishers have been killed or seriously injured in attacks carried out by extremists since 2013.
Almost all the attacks have been claimed by transnational Islamist extremist groups, including Al Qaeda affiliates and the Islamic State.
Officials here, however, say the killings are mostly the work of homegrown radical groups.
The killing of a Hindu ashram worker in northern Bangladesh on Friday was also claimed by the IS, according to the Site Intelligence Group, which monitors jihadi activity online.