"We're still trying to establish the motive for the killing and the identities of the gunmen," an investigator said.
Bersamin is a political ally of Gloria Arroyo, the Philippine president, who earlier in the week called for change in the country's political system.
The 62-year-old congressman was a lawyer and businessman and was the vice-chairman of both the House committee on dangerous drugs and the committee on trade and industry.
Soldiers and police officers have been on alert since Friday because of a demonstration organised by Roman Catholic bishops in Manila's main public park on Sunday to express opposition to moves by Arroyo's allies in congress to change the constitution.
Opposition from powerful religious groups forced the lower house of congress to abandon attempts this week to alter the charter and shift the country to a parliamentary system, which critics saw as an attempt to extend Arroyo's powers.
Church leaders said the gathering would still go ahead to express outrage.