The mother of a 2014 Peshawar school attack victim shows pictures of her son to a student victim (left) of the Bacha Khan University attack at a hospital in Charsadda, Pakistan, on Thursday. Photo: AFP

Pakistan observed a national day of mourning on Thursday for the 21 people killed when heavily-armed gunmen stormed a university in the country's troubled northwest on Wednesday, exposing the failings in a national crackdown on extremism.

Armed police were still deployed Thursday morning at Bacha Khan University in Charsadda, where students were targeted with grenades and automatic weapons.

Security forces remained  on alert, with police foiling a bomb attack at a bus station in Peshawar on Thursday morning.

Wednesday's assault, which was claimed by a faction of the Pakistani Taliban, bore a chilling resemblance to a December 2014 massacre at a school in Peshawar.

That attack triggered a crackdown on militants that had been credited with a palpable improvement in security.

Around 1,000 people attended the Thursday funeral of Fakhr-e-Alam, a university caretaker killed in the massacre.

"I want to tell the terrorists they can never win. They will lose, we will win, we the followers of peace and not terrorism," said Shah Hussain, father of the caretaker.

One of the wounded students died overnight, and his funeral was also held Thursday. The majority of the dead were ­buried on Wednesday in accordance with Muslim tradition.

Seven other survivors were in stable condition and were being treated in local hospitals, officials said.

Defiant authorities kept schools in Khyber Pakhtunkhwa­ Province open on Thursday.

"Militants want them shut down," provincial education minister Arif Khan said. "We wanted to send the message that education will continue."

Flags flew at half-mast on government buildings while a prayer ceremony was set to be held in Islamabad, where ­Pashtun students were also ­organizing a protest.

More than 200 athletes gathered alongside officials from the Pakistan Sport Board (PSB) at a complex in the capital­ ­Islamabad.

Prime Minister Nawaz Sharif has vowed a ­"ruthless" response to the massacre and ordered security forces to hunt down those behind ­Wednesday's attack.

Authorities arrested 50 suspects from the area surrounding the university, the district police chief said, as the military spokesperson said on Twitter that the attack had been masterminded by a Pakistani Taliban operative from Afghanistan.

Pools of blood and ­overturned furniture could be seen inside a hostel where the majority of the students died, while in a back alley outside, an old wooden plaque on the wall proclaimed "Heroes die young."