Opening a summit of the Organisation of the Islamic Conference (OIC) - the world's biggest Muslim body - in the city of Makka, Abdullah said the world's one billion Muslims were weak and divided.
"It bleeds the heart of a believer to see how this glorious civilisation has fallen from the height of glory to the ravine of frailty, and how its thoughts were hijacked by devilish and criminal gangs that spread havoc on earth," Abdullah said.
Saudi Arabia, home to 15 of the 19 al-Qaida hijackers who killed 3000 people in the United States on 11 September 2001, is also battling a wave of insurgent violence at home.
US critics have blamed the kingdom's strict al-Wahhabi school of Islam for fostering extremism, but Saudi officials say they are tackling the insurgents through a tough security crackdown and a campaign to win over their sympathisers.
Abdullah called for greater educational efforts to promote moderation.
"I look forward ... to the spread of a moderation that embodies the tolerance of Islam," Abdullah said.
The king was speaking at the start of a two-day summit in Makka of the 57-member OIC, convened to address what he said were grave dangers facing the Muslim nation.
Badawi (L) said Muslims cannot
remain in a state of denial
Countries represented in Makka include Iraq, where the war between the Iraqi and non-Iraqi fighters and the US-led forces in Iraq have killed thousands of people in attacks on US troops, the government they helped to install and Iraqi civilians.
Iran and Syria, under US pressure over nuclear plans and the assassination of a former Lebanese prime minister, are also attending, as well as Palestinians and leaders of several Asian and African states.
Malaysian Prime Minister Abdullah Ahmad Badawi also struck a solemn note, saying Muslims across the world were in a state of "disunity and discord" which he said was worse than any time in the 14 centuries of Islamic history.
"Muslims of the present age appear hopelessly divided," Badawi, whose country holds the OIC chair, told the opening session." ... We can no longer afford to be in a state of denial."
The Makka gathering also aims to revive the OIC, which has been largely ineffectual since it was set up 36 years ago, with the stated aim of recovering East Jerusalem from Israeli occupation after the 1967 Middle East war.
A 10-year plan presented to the leaders calls for better education, faster economic development, more trade, promoting religious moderation, and strengthening rights of Muslim women.