Bahraini protesters look at a teargas canister fired by police during a demonstration in Sanabis near Manama on February 14
(CNN) -- A man participating in the funeral procession for a protester slain during recent unrest in Bahrain was killed Tuesday, the second fatality in the small Persian Gulf country swept up by the dramatic political upheaval across the Arab world.
Authorities and activists says the victim was 31-year-old Fadhel Matrook -- one of thousands of people attending the funeral procession of Ali Abdulhadi Mushaima, according to Nabeel Rajab, president of the Bahrain Center for Human Rights.
Human rights groups say Matrook was shot by security forces using pellet guns. Activists have decried the use of those weapons at short range by Bahrain's security forces.
Bahrain's Interior Ministry said on its website that an investigation has been ordered to find out the reason behind the incident.
Mushaima, who died Monday, was shot protesting for human rights in the village of Daih, near Manama, Rajab said.
Al Jumri said another man was critically injured by pellet fire Tuesday during the funeral procession, and many others needed treatment for pellet wounds.
Interior Minister Lt. Gen. Shaikh Rashid bin Abdullah Al Khalifa "offered condolences and deep sympathy to the family of Ali Abdulhadi Mushaima on Monday," according to a statement on the Interior Ministry's website.
The main opposition party, al Wifaq, has suspended its participation in parliament because of the security forces' action.
"We decided to suspend our membership until further notice, after the aggressive attacks by the police on civilians demonstrating and carrying the kingdom's flag and calling for political and constitutional reforms," said party leader Abdul Jalil Khalil.
Rajab told CNN the security forces appear to have been taken by surprise by the size of the marches.
Another source said security forces were used to dealing with small groups of Shiite youths burning tires, but the diverse nature of the protests Monday -- in which demonstrators demanded constitutional reform and waved the Bahraini flag -- caught them off balance.
Bahrain is ruled by a Sunni royal family, but two-thirds of the population are Shiite. In recent years, younger Shiites have staged violent protests to complain about discrimination, unemployment and corruption.
The protest movement in Bahrain has been organized using social media websites such as Facebook and Twitter, in the same manner as the protests in Tunisia and Egypt.
Bahraini activists have created websites on which they've posted video of the unrest and the response by security forces.
The protesters called for a "Day of Rage" Monday to mark the 10th anniversary of the National Action Charter, which ushered in a new Bahraini constitution. Many Shiites say the constitution has done little to improve their condition.
Human rights activists say the protesters initially demanded reform and the introduction of a constitutional monarchy, but some are now calling for the removal of the royal family.
Bahrain is a pro-Western kingdom that is also the headquarters of the U.S. Navy's Fifth Fleet. Recently published diplomatic cables show that its ruler, King Hamad ibn Isa Al Khalifa, has been deeply suspicious of Iran's behavior in the region.
CNN's Jenifer Fenton contributed to this report