Three protesters have been killed in eastern Saudi Arabia by security forces, a Saudi activist said on Tuesday.

Tawfiq al-Saif, an activist, told Reuters that the government was sending a team of investigators to the city of Qatif in the Eastern Province to investigate the deaths.

Ali al-Felfel, 24, and a nine-year-old girl were shot dead in a demonstration in Qatif during the late hours of Monday, according to the activists, Press TV reported on Tuesday. 

Saudi forces also killed 19-year-old Nasser al-Mahishi in Qatif on Sunday night. 

On Monday, the protesters in Qatif seized an armored vehicle belonging to the security forces.

Protesters in the eastern city of Qatif and nearby town of Awamiyah continued to hold demonstrations against the House of Saud despite security forces' crackdown on the protesters. 

Earlier in the day, security forces opened fire on the protesters in Qatif, leaving several people dead and injured, according to activists. 

Thousands of Saudi demonstrators mourned the death of Mahishi. Mourners also chanted slogans against the U.S.-backed Saudi royal family. 

Mahishi was shot by regime forces while walking down a street in Qatif with his friends. He succumbed to his wounds on Monday. 

Similar demonstrations also took place in Awamiyah, where protesters burned tires and blocked main roads. 

A court in Saudi Arabia sentenced 17 men to prison sentences of up to 30 years on Tuesday for sedition and other offences, a lawyer for some of the defendants said.

"Myself, their families and judges whom we know on the bench are all shocked," defense lawyer Bassim Alim told Reuters.

He added that the judge had promised a written verdict in two to three weeks, at which time a 30-day window for lodging appeals would be open to the accused - who have been described by Amnesty International as proponents of peaceful reform.

Justice Ministry spokesmen were not available for comment.

Most of the group of activists, academics and lawyers were detained in 2007 after they met in the Red Sea port city of Jeddah to discuss potential political change in Saudi Arabia, an absolute monarchy governed by a strict form of Islamic law.

Amnesty International described the men in its 2011 annual report as "advocates of peaceful political reform".

They were charged, among other crimes, with attempting to seize power, incitement against the king, financing terrorism, electronic crimes, money laundering and trying to set up a political party, Alim said before the sentencing.

Saudi Arabia's east has been the scene of anti-government protests over the past months with demonstrators demanding human rights reform, freedom of expression and the release of political prisoners. 

Regime forces used heavy weapons including artillery and machine gunfire to disperse the protesters on Monday. Similar demonstrations were held in the nearby town of Awamiyah on the same day. 

Last week, Saudi security forces in Qatif arrested two people who were accused of taking part in demonstrations demanding reform and the release of political prisoners in the kingdom. 

Saudi authorities have prohibited public gatherings in the wake of months of anti-regime protests in several cities. Security forces have injured and arrested dozens of people in the country over the past few days. 

The arrests in Saudi Arabia have been carried out despite the fact that the kingdom is a party to the Arab Charter on Human Rights. Article 14 of the charter prohibits arbitrary detention.

Respondents to a Press TV poll believe the Saudi Arabian royal family will face internal conflict and be forced to leave power. 

According to the opinion poll, about 47 percent of respondents believe that the Saudi royal family will eventually step down. 

The poll indicates that around 45 percent of respondents are of the opinion that the House of Saud, which was installed by the United States and Britain, will continue to rule with the unwavering support of the U.S. 

About eight percent said that the House of Saud will step down from power in favor of a popular government.