(Adds details on instability in Egypt, background on past attacks)
By Shadia Nasralla
CAIRO, May 18 (Reuters) - A bomb wounded at least three people at a Cairo election rally for presidential frontrunner Abdel Fattah al-Sisi on Saturday, security sources said, underscoring Egypt's instability nearly a year after he toppled Islamist leader Mohamed Mursi.
A man riding on a motorcycle threw the homemade device in the direction of Sisi supporters at the street gathering of about 150 people in a north-eastern district of the capital, the sources said.
Former army chief Sisi, who was not present, is expected to win the election on May 26-27, after gaining the support of many Egyptians who backed his ouster of Mursi after mass protests against his rule.
Sisi has avoided public appearances during his election campaign, apparently for security reasons. He said in a television interview that there were two attempts on his life, but did not elaborate.
Saturday's attack was the first targeting a campaign event.
Sisi's only competitor in the election is leftist Hamdeen Sabahi who placed third in presidential elections in 2012.
Militant attacks have intensified and spread beyond Islamist strongholds in the Sinai Peninsula since Mursi's fall. Hundreds of police and soldiers have been killed.
Egypt's interior minister survived an assassination attempt in Cairo last year. He said earlier on Saturday that security forces were ready to ensure elections would be held in safety.
The security sources said it was not clear whether Saturday's attack had been aimed at the police or Sisi supporters but one of the wounded was a police officer.
Whatever the case, it highlights the persistent instability in the biggest Arab country three years after an army-backed popular uprising toppled autocratic president Hosni Mubarak and raised hopes of a brighter future.
In February, two Korean visitors were killed in South Sinai when militants bombed their bus, in another blow to the tourism industry, a pillar of the economy. Most high-profile attacks have been claimed by Sinai-based group Ansar Bayt al-Maqdis.
Security forces have devastated the Brotherhood, killing hundreds of Mursi supporters at a Cairo protest camp last August and arresting thousands of others.
But containing the most hardcore Islamist groups in the Sinai has proven far more difficult, and security officials say militants based along the border with Libya are also becoming a threat. (Reporting by Shadia Nasralla; Editing by Michael Georgy and Lisa Shumaker)