* Security forces say three linked to army commander killing
* U.S.-backed operation forced militants from strongholds
* Activists say two Yemenis killed by security forces in Aden (Adds defence ministry, killing of two activists)
ADEN, June 22 (Reuters) - Yemeni security forces have detained a local al Qaeda leader and two of his aides suspected of links to an attack that killed a top military commander earlier this week, a security source said on Friday.
Yemeni security forces have stepped up security across the country since a suicide bomber killed Major General Salem Qatan, the commander of military forces in southern Yemen, on Monday after his forces drove al Qaeda allies, Ansar al-Sharia, from their strongholds in the area.
A Yemeni security source said Sami Dayyan, who they believe is a local al Qaeda leader, was captured along with two other suspected militants while travelling by car to Lahej province from the port city of Aden.
The source said that explosives and suicide belts were found in the vehicle, which was stopped for a routine search outside Aden on Thursday evening. The three men were being questioned.
"They were in the process of preparing for operations similar to the one that killed Qatan," the source said.
The men had "entered Aden in previous days to prepare for the operation in which Qatan was assassinated," he said.
The Yemeni Defence Ministry confirmed the arrest in a statement sent over mobile telephones and described Dayyan as "a prominent al Qaeda leader".
Al Qaeda said in a statement on Thursday that one of its members had killed Qatan for his role in the U.S.-backed offensive that drove Ansar al-Sharia out of cities they had captured during last year's uprising that eventually forced President Ali Abdullah Saleh to step down.
Qatan's death highlighted the tenuous grip of Yemen's central authorities on the south despite weeks of a U.S.-supported campaign to crush the militants.
Residents of the al-Mansoura district of Aden said they had spotted Dayyan looking to rent a house in the neighbourhood a few days ago and chased him away.
In a separate incident, activists in a group calling for the secession of southern Yemen said on Friday that security forces shot and killed two young men in Aden during a march following the funeral of another man who was killed by a sniper on Wednesday.
They said paramilitary policemen guarding a square that had recently been cleared of protesters who had occupied it for a year to press for the restoration of the Marxist state that merged with North Yemen in 1990, opened fire on the marchers when they approached.
One young man was killed on the spot while the other died in hospital, they said.
A Yemeni security official confirmed that security forces opened fire on an unlicensed demonstration which he said was trying to force its way into the square. The official, who declined to be identified, made no mention of casualties. (Reporting by Mohammed Mukhashaf; Writing by Sami Aboudi; Editing by Robin Pomeroy)