Saudi police have taken extra precaution and are on high alert following violence that took place in the east of the country.
Saudi police put up checkpoints around the town of Qatif on Friday, after two people were killed and six others were wounded on Wednesday in an exchange of gunfire between security forces and what the Saudi interior ministry called criminals serving a foreign power.
Slogans condemning the royal family have appeared on walls throughout the Qatif.
Wednesday's deaths brought the toll to four people dead, with nine others wounded, since unrest erupted in in the region last week.
"These casualties have occurred due to the exchange of gunfire with unknown criminal elements who have infiltrated among citizens, and are firing from residential areas and narrow streets," the interior ministry said on Thursday.
The ministry denied that Shias had been killed by bullets fired by police in Qatif, an administrative unit of the province where a large Shia Muslim community resides.
"It [the ministry] warns whoever deludes himself about violating order that he will be deterred strongly, and that the security forces in the area are fully authorised to deal with the situation and end these criminal actions."
- Interior Ministry, Saudi Arabia
Echoing language it used after an attack on a police station in the Eastern Province last month, the ministry said: "The goal of those who provoke unrest is to achieve dubious aims dictated to them by their foreign masters."
The previous references to foreign meddling have been widely interpreted as Shia-dominated Iran, the Sunni-led kingdom's rival for influence in the Gulf.
Sunni Arab monarchies in the region saw Iran as the force behind unrest earlier this year in majority Shia Bahrain.
Iran has denied repeated accusations that it is trying to destabilise Bahrain.
It has also dismissed an alleged plot that US authorities said last month had implicated Iran's security agencies in a plan to kill the Saudi ambassador in Washington.
Saudi Arabia has avoided mass protests that have led to the ousting of four Arab leaders this year, reacting to the unrest in the region by promising to spend some $130bn on housing and other social benefits for its citizens.
Small-scale protests have taken place in the Eastern Province, where authorities have responded by deploying armed riot police and establishing checkpoints, activists said.
The province is connected by a 25km causeway to Bahrain, where Riyadh sent troops earlier this year to help the fellow-Sunni government crush protests led by Shia activists.
Saudi's Shia community complains of systematic discrimination, neglect in public spending and incitement against them in religious sermons and educational materials.
The kingdom, which is founded on an austere form of Sunni Islam and regards itself as the guardian of that faith, disputes this.
Officials point out that King Abdullah has appointed Shia officials to advisory government bodies in the kingdom.
A Saudi activist said earlier this week that three people had been killed by gunfire in the region, including two he said were hit by police bullets during a protest march.
Another activist said police had fired on protests in Qatif and the nearby town of Awamiya.
The ministry at that time said two people had died in incidents in the region, including a shooting at a police checkpoint near which tyres had been set on fire. It said it would investigate the incidents.