Sitra, Bahrain (CNN) -- Thousands of people took to the streets in Bahrain Thursday as the funeral took place for a 14-year-old boy whose death a day earlier sparked wide anger, witnesses said.
Clashes broken out overnight Wednesday into Thursday between Shiite Muslim protesters and police, after witnesses said they saw Ali Jawad al-Sheikh collapse after riot police fired a tear-gas round at him and other protesters in Sitra, southwest of the capital Manama.
But Nabeel Rajab, president of Bahrain's Center for Human Rights, who was at the funeral, said the procession had remained calm, with no outbreaks of violence.
He told CNN crowds of people had gathered from the early morning but police had pulled out from the entire area, using helicopters instead to monitor the situation.
Rajab predicted larger protests demanding political reform would take place later Thursday.
"We expect to see protesters out tonight. The February 14 Movement called for a protest tonight in Manama and places around Manama," he said, adding that tensions had been building in the past three to four weeks, as people lost hope of achieving a political solution to the country's problems.
Meanwhile, government officials say they are investigating the death and Bahrain's Interior Ministry has offered a 10,000 Bahraini dinar ($26,400) reward for information leading to the arrest of his killer, state news agency BNA reported Thursday.
The Interior Ministry has said no clashes were taking place at the time the boy was injured, saying that the last reported incident of unrest in the area was around 1:15 a.m. Wednesday.
And a police chief said Thursday that the hospital officials who informed the police they had received the boy's body did not give any details about the incident or where the body was found, BNA reported.
Bahraini officials said Thursday that the doctor who carried out an autopsy on the body concluded that the cause of death was an injury sustained behind the neck, where there were fractures causing bleeding around the spinal cord.
Blood tests by the forensic laboratory did not show any sign of tear-gas exposure, BNA cited public prosecution chief Osama Al Asfoor as saying.
Bahrain's interior ministry issued a news release later Thursday, citing the coroner's report in discounting a fatal hit from a tear-gas canister or rubber bullet.
"The coroner's report indicates that the markings on Ali's neck are not consistent with being hit with a tear gas canister or rubber bullet as some have claimed," the release said. "The markings were too large and suggest that he was hit with a larger object. Further, the coroner's investigation shows that no tear gas was found in his lungs."
The release said the youth died "as a result of a serious blow to the back of the neck ... that culminated in a blood clot in the brain, which eventually took his life."
The release urged "calm in response to the tragic death of a 14-year-old boy" and asked for the three unknown men who brought the boy's body to a hospital "to make themselves known so that they can explain how they found Ali and offer an explanation of the events of the morning that (led) to his death."
Speaking ahead of the burial, Bahraini pro-democracy activist Zainab Alkhawaja, in Sitra, told CNN there was a feeling of anger in the streets, with people chanting slogans calling for the fall of the regime and its leader, King Hamad.
"People are more upset and angry than I've seen at (a) funeral here before -- I think because he was so young," she said.
Public prosecutors have begun examining the incident, according to a statement from Bahrain's Information Affairs Authority, and it would also fall under the authority of the Independent Commission of Inquiry set up by King Hamad in June to investigate reports of human rights abuses.
Clashes broke out Wednesday on the first day of the Eid al-Fitr festival for many Shiites in Bahrain, one of several countries embroiled in anti-government protests across the Middle East and North Africa this year.
By early Thursday, they had spread to most Shiite villages and parts of Manama.
Police responded heavily to street fights in three districts near the Pearl Roundabout, the epicenter of anti-government protests that began in February, witnesses said. Bahrain demolished the landmark Pearl Monument at the center of the plaza in March after it became a symbol of the demonstrations.
Protests have lingered for months despite a crackdown by Bahrain's Sunni monarchy, backed by troops from Saudi Arabia and the United Arab Emirates. More than 30 people have been killed the crackdown, in which activists say Bahraini security forces used live ammunition.
Opposition groups say more than 1,000 people -- mainly Shiites -- have been detained and more than 2,000 have lost their jobs for allegedly taking part in the protests.
Bahrain is a close ally of the United States and houses the headquarters of the U.S. Navy's 5th Fleet. Its rulers blame Shiite-ruled Iran for stirring up trouble among its Shiite majority, but opposition leaders and Iranian officials deny the allegation and many Western powers have dismissed it.
The boy's father, Jawad al-Sheikh, told CNN Wednesday that when he went to the morgue, his son was "hard to recognize because he was covered with blood." He appealed for human rights groups to take steps against Bahrain's leaders, saying, "I lost my child. ... He does not deserve this destiny."
Tuesday, Rupert Colville, a spokesman for the U.N. High Commissioner for Human Rights, said the situation in Bahrain remains "tense and unpredictable," with small protests still being repressed.
The United Nations has demanded that Bahrain release prisoners detained for exercising freedom of speech.
CNN's Mohammed Jamjoom contributed to this report.