Those who surrender will be "secure" but those who do not will be subjected to a fierce crackdown, King Fahd said in a speech on Wednesday and delivered on his behalf by Crown Prince Abd Allah bin Abd al-Aziz on state television.
While officials have repeatedly urged Islamist fighters and dissidents to repent, it was the most concrete proposal to be made in public and followed the killing of the kingdom's al-Qaida chief after his group stepped up attacks against foreigners.
A prominent Islamist who has been engaged in efforts to broker an end to the violence urged the government to follow up its initiative by defining a "clear mechanism showing the way for those who wish to return to the fold" and by releasing detainees not found guilty.
"All those affiliated to this group which wronged itself and who were not captured ... have a chance to return to God and engage in soul-searching," King Fahd said.
One month limit
"Those who turn themselves in of their own will within a month of this speech will be secure in the security of God as far as they are concerned, and will be treated according to God's law pertaining to the rights of others."
"Those who turn themselves in of their own will within a month of this speech will be secure in the security of God as far as they are concerned, and will be treated according to God's law pertaining to the rights of others"
King of Saudi Arabia
This suggested the state would drop all charges against those who surrender but that they would still have to obtain a pardon from people harmed by their acts if they are to avoid prosecution.
"We announce for the last time that we open the door of pardon, return to right and applying the righteous rule of sharia (Islamic law) to anyone who deviated from the right path and committed a crime in the name of religion," the Saudi monarch said.
'We are not weak'
"Everyone knows that we are not saying this out of weakness, but as an option to those, and so that we will be excused as government and people [if we crack down] since we opened the door of return and security.
"The rational who will take up [the offer] will be secure. As for those who spurn it, by God, our forbearance will not prevent us from striking with the full force we draw from our reliance on God."
The address came five days after security forces shot dead al-Qaida's local chief, Abd al-Aziz al-Muqrin, and three associates who had beheaded an American hostage in the latest of a spate of attacks on foreigners in the kingdom.
Security forces have yet to recover the body of aeronautics engineer Paul Johnson, shown beheaded in photos posted on websites by Muqrin's group, which calls itself Al-Qaida in the Arabian Peninsula."