Saudi police on Thursday deployed in force in a poor neighbourhood of Riyadh after rioting by illegal migrants the previous night left one person dead, an AFP correspondent said.
And in the commercial capital Jeddah, security forces dispersed a large protest by illegal migrants and arrested five Ethiopians suspected of organising the demonstration, a police spokesman said.
Protests are banned in the ultra-conservative kingdom, where tensions have been spiralling since the authorities began rounding up thousands of illegals following the expiry on November 4 a final amnesty for them to formalise their status.
Among them are foreigners who overstayed their visas, pilgrims who have sought jobs, and migrants working under one sponsor trying to get jobs elsewhere.
Clashes on Wednesday between Saudis and illegal migrants in the southern Riyadh neighbourhood of Manfuhah left one Sudanese national dead and 17 people injured, police reported.
Riyadh had already announced the deaths of a Saudi and two foreigners in Manfuhah on Saturday, while the Ethiopian embassy said three of its nationals had died in clashes.
Police vehicles on Thursday jammed the main street of Manfuhah, which is populated mainly by Africans, the AFP correspondent said.
For the fifth day in a row, long lines of illegal migrants queued in Manfuhah for buses to take them to repatriation centres.
The heavy security force presence brought commercial activity in the neighbourhood to a near-halt, with many shops and restaurants catering to Indians, Pakistanis and Ethiopians remaining shut.
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"I have lived in Saudi Arabia for eight years" without papers, said one Ethiopian in his 30s who declined to give his name.
He said he had entered the kingdom illegally after travelling by boat from Yemen.
Another Ethiopian, Ahmad Mumen, arrived three years ago, also clandestinely.
"I have no papers," said the youth, adding that he had found night-time employment in a shop.
Most of the women queueing for the buses said they had found employment as domestic workers.
Many illegal migrants living in the area have turned themselves in to police to be deported.
Expatriates account for a full nine million of the oil-rich kingdom's population of 27 million.
The lure of work, even in low-paid jobs as domestics or construction workers, has made Saudi Arabia a magnet for migrants from Asia and poorer Arab states.
Nearly a million migrants -- Bangladeshis, Filipinos, Indians, Nepalis, Pakistanis and Yemenis among them -- took advantage of the amnesty to leave.
Another roughly four million were able to find employers to sponsor them.
Despite its huge oil wealth, Saudi Arabia has a jobless rate of more than 12.5 percent among its native population, a figure the government has long sought to cut.