More than 200,000 Yemenis took to the streets on Tuesday demanding the repeal of an immunity for ousted president Ali Abdullah Saleh, in the largest protest since the veteran leader stepped down in February, organisers said.

"The number of participants has exceeded 200,000," said a member of the organising committee, while another said protesters covered around five kilometres (3 miles) along Al-Zubairy main street of the capital, with others still joining.

"Protesters are still crowding the starting point at Bab al-Yemen while the demonstration has covered half of Al-Zubairy Road," he said requesting not to be named.

The protest, which had been called for last week by the Committee of Youth of the Peaceful Revolution, came hot on the heels of an explosion that targeted the Defence Minister Mohammed Nasser Ahmed.

He survived the attack but 12 people were killed, including seven of his guards.

Protesters want to bring Saleh and his aides to justice over the killing of demonstrators during a year of demonstration against his rule that lasted 33 years.

Although he stepped down, Saleh is believed to be working behind the scene to undermine the political transition in the country. Some go as far as accusing him of instigating violence.

"If the immunity remains, Saleh and his followers will be encouraged to continue in sabotaging oil (pipelines), and electricity grid, robbing, and supporting terrorism," declared a banner carried by demonstrators.

Under the Gulf-brokered peace deal, Saleh abandoned his post to his then deputy Abdrabuh Mansur Hadi after a national unity government was formed, while in return he was given immunity against prosecution.