Members of militant Islamist groups were reacting to word from Washington that the al Qaeda leader was killed in a shootout with U.S. forces on Sunday.
"Oh God, please make this news not true... God curse you Obama," said one message on an Arabic language forum. "Oh Americans... it is still legal for us to cut your necks."
U.S. forces killed bin Laden in a raid on his hideout in Pakistan, President Barack Obama said, ending a nearly 10-year worldwide hunt for the mastermind of the Sept. 11 attacks.
His killing, in a mansion outside of the Pakistani city of Islamabad, dealt a symbolic blow to the global militant network, although Islamist forum posters said the strike would not change their commitment to fighting Western powers.
"Osama may be killed but his message of Jihad will never die. Brothers and sisters, wait and see, his death will be a blessing in disguise," said a poster on another Islamist forum.
Another forum member pointed to the irony of bin Laden's location, contrasting with long-time rumours that he was hiding in caves. "So after 10 years of hiding in mountains, he ends up getting killed in a mansion outside of Islamabad. Interesting."
But the prevailing sentiment was one of grief.
A poster on the Arabic-language Ansar forum said, "God's revenge on you, you Roman dog, God's revenge on you crusaders... this is a tragedy brothers, a tragedy."
Online forums for militant Islamists have been the key means of passing messages from bin Laden and his second-in-command Ayman al-Zawahiri, as well as al Qaeda's regional branches, such as al Qaeda in the Arabian Peninsula in Yemen.
"Forums play a role in communications and ideas for al Qaeda followers, similar to the way Facebook and Twitter were used by democracy protesters in the Arab revolutions of 2011. It's a powerful medium," said Theodore Karasik, a Dubai-based security analyst for the INEGMA group.
Militants also commonly use the forums to pass tips for making explosives, discuss methods of attacks or voice their opinions on world events in relation to Islamist views.
Many argued on Monday they could not believe the news of Osama bin Laden's death until it was confirmed online by al Qaeda's official news outlet, al-Fajr.
"Everyone try to be calm and pray and wait for a response from our brothers at al-Fajr center to learn the accuracy of the news," a message on Ansar news said.
Others doubted the authenticity of photos circulating on the Internet depicting the face of bin Laden after his death. They argued previous pictures of him alive looked older, and his beard greyer, than the picture which some claimed were of his corpse.
But on the Islamic Awakening forum, some suggested bin Laden's death should be accepted and a new leader found.
"Why can't people admit he was killed? He is a human being, not a prophet. Another man will replace his shoes, it's easy."
Others ridiculed the celebrations playing out in the United States, where crowds cheered and waved flags outside the White House and at New York's "Ground Zero", site of the World Trade Center twin towers felled by hijacked airliners on Sept. 11, 2001.
"Please let them celebrate, they are celebrating their own end," said Abu Aziza on the Islamic Awakening forum. "Oh Allah destroy this nation for their hatred and enmity toward your deen (religion)."