Second-year engineering student Abrar Fahad of capital Dhaka's Bangladesh University of Engineering and Technology (BUET) was found dead in a dormitory on Monday morning with bruises on the body.
Police have arrested 15 suspects, most of them linked to the Bangladesh Chhatra League - the student wing of Prime Minister Sheikh Hasina's ruling Bangladesh Awami League, Efe news reported.
"(The) victim's father (has) accused 19 people of the killing. We have continued our drive to arrest them," said Masudur Rahman, a spokesperson of Dhaka Metropolitan Police.
Protesters allege that Fahad was beaten to death after a group of student activists took him away on Sunday night.
That was a day after he wrote a Facebook post criticizing the government's water sharing agreement that allows neighbouring India to draw water from the Feni River. The deal was signed during Hasina's recent visit to India.
"He (Fahad) was a very conscious person, so he tried to give his opinion on Facebook. Maybe the ruling party considered him a threat," said Towhidul Islam, a roommate.
Islam said they were not alarmed when they heard that some activists had taken him away in full view of people.
"On our campus, junior students are often taken for ragging (by their seniors). So we were not that worried," said Islam.
An autopsy has shown that his body was covered with bruises.
"We found bruises in many parts of his bodies and hemorrhages under his skin. So we assumed he died because of beating," said Sohel Mahmud, the head of the forensic department of Dhaka Medical College Hospital.
A CCTV footage from the dormitory area has recorded some seven to eight people taking Fahad inside a room, according to a student, who saw the recording. Four others are later seen taking him out.
"They are all ruling party men," the student, requesting anonymity, told EFE.
The protest began on Monday and have continued since.
The protesters are demanding that the accused be expelled from the university and the trial in the murder fast-tracked.
"We want justice," said protester Antora Madhuri, urging the authorities to ban political activities on the campus.
Prime Minister Hasina has promised to bring the culprits to justice regardless of their political affiliations.
"Criminals are criminals and they would be brought to justice," Hasina told reporters on Wednesday.
The United Nations on Wednesday also condemned the killing.
"Freedom of speech is a human right and nobody should be harassed, tortured or killed for exercising it," the UN said in a statement.
Hasina's government is widely criticized for its crackdown on dissent. It passed a controversial internet law last year that has provisions of life imprisonment for revealing state secrets and publishing "false or distorted" information.
( With inputs from IANS )