Tens of thousands of Yemenis have squared off in street protests for and against the government during an opposition-led "day of rage", a day after President Ali Abdullah Saleh offered to step down in 2013.

By early morning, the number of anti-government activists in Sana'a had reached more than 20,000, the biggest crowd since a wave of protests hit the Arabian Peninsula state two weeks ago, inspired by demonstrations that toppled Tunisia's ruler and threaten Egypt's president.

But an equally large pro-Saleh protest was gathering steam. Supporters of the president who has ruled Yemen for more than three decades were driving around the capital urging Yemenis over loudspeakers to join their counter-demonstrations.

Saleh, eyeing the unrest spreading in the Arab world, indicated he would leave office when his term ended in 2013, and promised his son would not take over the reins of government, among a host of other political concessions.

It was Saleh's boldest gambit yet to stave off turmoil in Yemen, a key ally of Washington against al-Qaida, as he sought to avert a showdown with the opposition that might risk sparking an Egypt-style uprising in the deeply impoverished state.

The stakes are high for Yemen, on the brink of becoming a failed state, as it tries to fight a resurgent al-Qaida wing, quell southern separatism, and cement peace with Shia rebels in the north, all in the face of crushing poverty.

Yemen's biggest opposition party, the Islamist Islah, welcomed Saleh's initiative but snubbed a presidential appeal to call off protests. Yet anti-government protesters appeared to lack consensus, with some calling for Saleh to leave, while others wanted him to prove he would act on his promises.

Further anti-government protests spread across Yemen, including in the town of Taiz, where Saleh once served as military governor, as well as in flashpoint southern towns where a separatist movement has grown increasingly active, The Guardian reported.