Saudi authorities have asked militant Islamists to surrender following the arrest of the suspected mastermind of the co-ordinated suicide bombings in Riyadh in May.
Thirty-five people died in the 12 May bombings
Ali Abdul Rahman al-Ghamdi, also known as Abu Bakr al-Azdi, is being intensely questioned after surrendering to a Saudi assistant minister of the interior on Thursday.
"All the wanted persons should surrender themselves. That is the only way because the security forces will reach them, and it is better for them to surrender themselves," the Saudi Interior Minister, Prince Nayef bin Abdul Aziz, told a Saudi newspaper.
Western intelligence sources have told the BBC that Ali al-Ghamdi was "al-Qaeda's top operative in Saudi Arabia" - and his arrest was a major blow to al-Qaeda's operations in the country.
If it turns out to be true, we will be very, very pleased that this terrorist has been brought to justice
US Secretary of State
Mr Ghamdi was number two on the list of most-wanted suspects in connection with the 12 May attacks.
Thirty-five people, including nine bombers, died in the attacks on compounds housing Americans and other westerners in Riyadh.
The US and Saudi authorities blame the al-Qaeda network for the bombings.
The Saudi interior minister said Mr Ghamdi had surrendered himself to Prince Mohammad bin Nayef in Jeddah after dawn prayers," Okaz newspaper reported.
Prince Nayef said a total of 50 people had now been arrested since the bombings at three Western compounds in Riyadh, which killed eight Americans.
"This is a major arrest and a huge victory in the war against terrorism," Prince Bandar bin Sultan, the Saudi ambassador to the US said in a statement.
US Secretary of State Colin Powell told reporters that if the reports were true, Washington would be "very, very pleased that this terrorist has been brought to justice".
Saudi Arabia's grand mufti, the highest religious authority in the kingdom, has also asked Saudis not to shelter fugitive militants.
"We must never protect those who are behind an incident in the country, but denounce them," Sheikh Abdulaziz al-Sheikh told Al-Hayat newspaper.
Mr Ghamdi fought in Afghanistan and according to US officials was present at the battle of Tora Bora, although he left the scene before heavy bombing started in December 2001.
He is thought to be about 30 years old and reportedly studied Islam in Saudi Arabia and Afghanistan.
US officials also say he has been linked to Saif al-Adel and Abu Mohammed al-Masri, two of the most senior al-Qaeda operatives who remain at large, the Associated Press news agency reported.
Both are thought to be hiding in Iran.
Mr Ghamdi was also an associate of Khalid Sheikh Mohammed, the alleged 11 September mastermind now in US custody, the officials said.
Ghamdi is suspected of links with Khalid Sheikh Mohammed
Most of the suicide bombers responsible for the attacks on the United States on 11 September 2001 were Saudi nationals, and Riyadh has been criticised for not doing enough to fight Islamic radicals.
The BBC's Justin Webb in Washington says this arrest will help Saudi Arabia to convince the Bush administration that every effort is being made.
Security in Saudi Arabia has been high since the suicide bombings in Riyadh.
Earlier this month Saudi Arabia identified 12 bombers who were behind the attacks, through extensive investigations and DNA testing.
Five of the names came from the list of 19 suspected al-Qaeda members who escaped arrest a few days before the attacks.