Pakistani authorities said they had arrested 14 suspects in connection with the alleged trafficking of migrants and police said they were searching for other suspects.
A statement from Prime Minister Shehbaz Sharif's office said the Pakistani government had ordered a high-level inquiry into the human trafficking network thought to be involved.
National flags were at half-mast on all official buildings on Monday in Pakistan, which was home to some of those who drowned last week.
The police report said one of those arrested had admitted to sending three men onto the boat, which had a capacity for 300-350 people, and had charged them each up to three million Pakistani rupees ($10,452.96).
The assessment 800 people were on board came from initial investigations, police officer Riaz Mughal said.
"We learnt from two survivors, the arrested suspects and the bereaved families that the boat was carrying around 750 to 800 people," Mughal told Reuters.
Witness accounts had placed the number on board at between 400 and 750 people and Greek authorities have said 104 survivors and 78 bodies have been brought ashore.
One of the suspects arrested in Pakistan said his own son had been on the boat, and was missing, the report said.
It also said the main suspect behind a smuggling network spanning Libya, Pakistan and Greece, was based in Libya.
Mughal said at least 21 of those who died last week came from the Kotli district in the Pakistani-administered part of the Himalayan Kashmir region, from which people have left for Europe for many years.
Senior Regional Police Officer Tahir Mahmood, based in Muzaffarabad, Kashmir's capital, said the police were hunting further suspects in addition to those arrested. He did not specify how many.
($1 = 287.0000 Pakistani rupees)
