* Second autopsy showed breathing passages clogged
* Protests over death planned for Friday
CAIRO, June 23 (Reuters) - A second autopsy on the body of an Egyptian activist whose death has become a rallying point for opponents of the government's emergency law to stifle dissent showed on Wednesday he died from asphyxia, not a police beating.
Rights groups said Khaled Mohammed Said, 28, was beaten to death by undercover police in the port city of Alexandria on June 6 after he posted an Internet video, which his family said showed police sharing the profits of a drug bust.
Supporters have called for silent protests across the country on Friday to mark his death. Mohamed ElBaradei, the former U.N. nuclear watchdog chief who might challenge for the presidency next year, has said he will take part.
The Interior Ministry had said Said died after choking on drugs he swallowed before police approached him, but Egypt's attorney general last week ordered a new autopsy because of the public outrage over the finding.
Conducted by the state-run department of forensic medicine, the autopsy concluded that Said's death was brought on by "the clogging of the breathing passages by a foreign object identified as a plastic roll containing marijuana".
The attorney general told a new conference, where he presented the report, that his body displayed signs of injuries resulting from a "collision with solid objects", but such injuries were not the cause of death.
Said's death has become a lightning rod for activists and government opponents who blame police abuse on the emergency law. In place since the assassination of President Anwar Sadat in 1981 and renewed until 2011, it allows arrests without charge and limits public gatherings.
The United States has urged Egypt to repeal the law. Egypt says the focus of the law is security and drug cases, but critics argue is it used to silence opposition.
A Facebook group memorial page called "We are Khaled Said" has drawn some 130,000 members. It calls on Egyptians to dress in black and stage a silent protest for an hour on Friday.
So far, activists in eight provinces, including Alexandria and Cairo, have announced they will protest simultaneously from 6:30-7:30 p.m. ElBaradei said he would join the protest in Alexandria. Up to 8,000 Egyptians wearing black protested along the corniche in Alexandria last week. Some recited verses of the Koran and Bible.
On Sunday protesters tried to march in downtown Cairo against police brutality and in solidarity with Said, but the gathering was swiftly dispersed after state security men beat and detained demonstrators. (Reporting and writing by Marwa Awad; Editing by Alison Williams)