(CNN) -- Three people were taken to the hospital Thursday after Saudi security forces fired on scores of protesters in the city of Qatif, according to two witnesses and an activist.
The protests took place one day ahead of a planned "Day of Rage" in the Middle Eastern country.
Defying a Saudi government ban on all kinds of public demonstrations, more than 100 people in the predominantly Shiite city in eastern Saudi Arabia urged authorities to release Shiite prisoners, the witnesses and activist said.
At some point, the witnesses said Saudi security forces shot to disperse the crowd. It was unknown if the forces fired rubber bullets or live ammunition. Those injured were taken to Qatif Central Hospital for treatment, the activist and witnesses said.
The witnesses and activist asked not to be named because they feared reprisals.
Ali Ahmed, the director of the Washington-based human rights advocacy group Institute for Gulf Affairs, who said he's been in touch with protesters, said a 16-year-old demonstrator was hurt in the hand, while an 18-year-old was injured in the foot. He claimed that a third unidentified person was more seriously hurt with injuries to the abdomen.
Ahmed said he had contacted the U.S. State Department with his concerns about the Saudi security forces' actions.
The previous day, a similar number of protesters took to the streets at Qatif. Human Rights First Society President Ibrahim Al-Mugaiteeb said that police kept a watchful eye, but did not intervene.
A Saudi interior ministry spokesman did not respond to repeated calls to discuss the incident, and other Saudi officials likewise had no comment on the matter.
Flanked by U.S. National Security Advisor Tom Donilon, Deputy National Security Advisor for Strategic Communications Ben Rhodes acknowledged that U.S. authorities were aware of reports that Saudi Arabian police had fired on protesters.
"What we have said to the Saudis and to all the people of the region is that we're going to support a set of universal values in any country in the region," Rhodes said, adding that the message has been "articulated in public and in private" to the Saudi government. "And that includes the right to peaceful assembly, to peaceful protest, to peaceful speech."
The incident Thursday came as a precursor to Friday's planned "day of rage" in Saudi Arabia, though longtime observers of the kingdom remained skeptical that it would make a major impact.
"I don't think any protests that happen tomorrow will be destabilizing to the country," said Christopher Boucek, a Saudi expert with the Carnegie Endowment for International Peace.
Prominent blogger Ahmed Al-Omran said the Saudi government remains unresponsive to people in the streets.
"I don't think they're really in touch with the people," he said.
Several of the recent protests -- including those in Qatif on Thursday -- have been driven by Shiite Muslims, who are a minority in the Sunni-dominated Arab kingdom.
The unrest brewing in parts of the Middle East and North Africa has appeared to get Saudi leaders' attention, with King Abdullah announcing a series of sweeping measures late last month aimed at relieving economic hardship.
That said, Saudi authorities have done little to change their tack against demonstrators, even authorizing security forces to "take all measures against anyone who tries to break the law and cause disorder."
Last week, about 24 protesters were detained in Qatif, as they denounced "the prolonged detention" of nine Shiite prisoners held without trial for more than 14 years, Amnesty International said.
Police kicked and beat three protesters with batons in what was an apparent peaceful demonstration, Amnesty said in a statement.
"The Saudi Arabian authorities have a duty to ensure freedom of assembly and are obliged under international law to allow peaceful protests to take place," said Philip Luther, deputy director of the human rights group's Middle East and North Africa program.
"They must act immediately to end this outrageous restriction on the right to legitimate protest."
There was no immediate reaction from the Saudi government to the Amnesty statement.
The protests in the majority Sunni kingdom have followed similar demands across the Arab world for more freedom and democracy.
Rights activists have been advocating the right to protest for months in the kingdom, but they have been denied permission to assemble.
Lately, grass-roots ferment mirroring the unrest across the Middle East and North Africa has emerged, with a Facebook group calling for days of rage and Shiites taking to the streets. Activists have been calling for reform and the release of people jailed without charge or trial.
Amnesty said the recent detentions came a week after a prominent Shiite cleric, Sheikh Tawfiq Jaber Ibrahim al-'Amr, was arrested after a sermon calling for reforms in Saudi Arabia. He was released without charge Sunday.
Most of the protesters are believed to be held in a police station in Dhahran, an eastern city. Among them are activists who have protested arrests and discrimination against the minority Shiites.
"The Saudi authorities must investigate reports of beatings of protesters by security forces. They should also ensure that those detained are either charged with recognizable offenses and tried fairly or released," Luther said.
"While in detention they must be protected from torture and other ill-treatment and given regular access to their family, lawyers and medical staff."
The Shiite activists in "prolonged detention" have been held in connection with the deadly 1996 bombing of a U.S. military complex in Khobar, in which 20 people were killed and hundreds injured.
"According to reports, they were interrogated, tortured and denied access to lawyers together with the opportunity to challenge the legality of their detention," Amnesty said.
CNN's Elise Labott contributed to this report.