"The Kingdom of Saudi Arabia, its people and government, stood and stands today with its brothers in Egypt against terrorism," he said in a message read out on Saudi television, in an apparent reference to continuing clashes between the Muslim Brotherhood and police.
"I call on the honest men of Egypt and the Arab and Muslim nations... to stand as one man and with one heart in the face of attempts to destabilise a country that is at the forefront of Arab and Muslim history," he added.
Hundreds were killed in Egypt this week when the security forces cleared protest camps set up by the Muslim Brotherhood to demonstrate against the military's ousting of President Mohamed Morsi last month.
Saudi Arabia was a close ally of former President Hosni Mubarak — the veteran strongman brought down by a popular uprising in 2011 — and it fears the spread of Muslim Brotherhood ideology to the Gulf monarchies. It pledged $5 billion in aid to Egypt after the Islamist Morsi was ousted.
"All those who meddle in Egypt's internal affairs are inflaming strife," he said, adding that the North African country faces "a conspiracy of plotters" trying to strike at its unity and stability.
Reporting by Angus McDowall in Riyadh, Sami Aboudi and Maha El Dahan in Dubai