Yemen's President Ali Abdullah Saleh has returned to the country from Saudi Arabia three months after surviving an assassination attempt.
Yemen TV said President Saleh arrived in the capital, Sanaa, by private plane at dawn.
Hours after his arrival, a presidential official said Mr Saleh was calling for a "truce and a ceasefire".
He went to Saudi Arabia in June for treatment following a rocket attack on the grounds of the presidential palace.
President Saleh, who has been in power for more than 30 years, has faced months of protests urging him to quit.
Correspondents say his return raises the risk of all-out civil war.
"The president calls on all political and military parties to achieve a truce," said the official, quoted by AFP news agency.
"There is no alternative to dialogue and negotiations to end the bloodshed," he added.
Later, the US urged President Saleh to begin a transfer of power and arrange presidential elections.
"The Yemeni people have suffered enough and deserve a path towards a better future," White House spokesman Jay Carney said.
In Sanaa, anti-government protesters and supporters of President Saleh held rallies after Friday prayers.
An al-Jazeera correspondent in the city says they have taken place within a few miles of each other, although it appeared many anti-government protesters had stayed at home for fear of violence.
Protest organiser Mohammed al-Asl said Mr Saleh's return would lead to "an escalation of violence".
"But let him come back - we want him to come back and be tried for his crimes," he added.
Activists have been camped out in an area of Sanaa dubbed Change Square since January, demanding an end to President Saleh's rule.
Clashes in the capital have recently intensified as elite Republican Guards, led by President Saleh's son Ahmed, fight running battles with army units that have defected to the opposition and tribal fighters who support the protesters.
More than 80 people, mostly unarmed protesters, have been killed since Sunday.
Doctors say at least one person was killed overnight and many others wounded when mortars landed in Change Square. Other reports said four people were killed in the northern al-Hasaba district on Friday.
Correspondents in Sanaa reported a surge in artillery and machine-gun fire as news of President Saleh's return broke. As least some of the gunfire was celebratory, they said.
Yemeni state TV broadcast old footage of Mr Saleh and played national songs as it reported his return.
Protest leader Abdel-Hadi al-Azizi said President Saleh's return "means more divisions, more escalation and confrontations".
"We are in a very critical escalation," he told AP news agency.
Hopes of a lasting ceasefire to enable a reform process to take place in which President Saleh would yield power are fading.
On Tuesday the government agreed a truce following negotiations with Western envoys. However, the ceasefire broke down hours later.
The UN High Commissioner for Human Rights, Navi Pillay, warned on Thursday that Yemen was at a "dangerous crossroads".
Gulf Co-operation Council (GCC) chief Abdulatif al-Zayani visited Yemen on Tuesday to try to broker a deal to the crisis but left the next day empty-handed.