DHAKA (Reuters) - Police in Bangladesh have arrested six suspected militants, including the chief of the outlawed Jamaat-ul Mujahideen group, as authorities stiffen a crackdown on hardline Islamists.

Bangladesh, a Muslim-majority nation of 160 million people, targeted the militants after machete-wielding attackers this year hacked to death three online critics of religious extremism, including American blogger Avijit Roy.

Police also seized explosives and other bomb-making materials during overnight raids on the militants' hideouts in the capital, Monirul Islam, joint commissioner of the detective branch of Dhaka police, said on Tuesday.

"During the raids we arrested six militants, including the head of Islamist outfit Jamaat-ul-Mujahideen," he added.

Indian security officials uncovered a plot against Bangladesh Prime Minister Sheikh Hasina in October, after two members of the group were killed in an explosion while building bombs in the state of West Bengal, which borders Bangladesh.

The banned Jamaat-ul-Mujahideen was thought to have been lying low after facing action by authorities following its detonation of nearly 500 bombs across the country almost simultaneously on a single day in 2005.

Its militants later carried out attacks on several courthouses, killing 25 people and injuring hundreds.