"They were firing indiscriminately with rubber bullets and tear gas," he said from his hospital bed.
One man was in serious condition in hospital after being struck by a security vehicle.
Another man was treated after being shot by a rubber bullet while another was wounded in the face by police buckshot.
The authorities said security forces only acted against demonstrators destroying public property.
"Some 500 men gathered in Jedhafs region, then marched to a main street and started to smash and sabotage public property and private possessions and set fire to trash cans, blocked the main road and then set fire to the market," said a statement by the police chief of the northern province.
The interior ministry said protesters had thrown firebombs and stones at government forces and had injured one policeman.
Abdul-Jalil al-Singace, spokesman of the Islamist Haq Movement for Liberty and Democracy organisation, said that Jassem's death could lead to new protests by Shia like those that rocked the country in the 1990s and resulted in 40 deaths.
"Jassem's death is the spark and a beginning for a new intifada [uprising]" he said.
"There will be a series of protests against Jassem's death and to highlight the regime's human rights violations."
Monday's demonstration was to mark the death 10 years earlier of a man during another round of Shia protests over discrimination.
Shia Arabs form the majority of the population in Bahrain and there have been several outbursts of violence in recent years against perceived discrimination by the country's Sunni ruling family.