Source: BBC, By Ginny Hill , Chatham House
When the ripple effect of the Arab Spring spread to Yemen earlier this year, it sparked a popular youth-led revolution and a parallel power struggle between three rival factions.
These three factions - President Ali Abdullah Saleh's family, the Ahmar family and leading army general Ali Mohsin - all belong to a privileged elite at the heart of Yemen's regime.
But pressures generated by successive weeks of street protests throughout the spring, combined with their existing suspicions and jealousies of one another, forced long-standing rivalries into the open.
In March, following a sniper attack on the protesters' camp in Yemen's capital, Sanaa, Gen Ali Mohsin broke ranks with Mr Saleh.
In May, fighting broke out between Mr Saleh's family and the Ahmar family in the Hasaba district of Sanaa when the president refused to sign a Gulf-backed transition deal. Clashes were abruptly halted after a botched bomb plot to kill the president left him badly injured.
Mr Saleh was evacuated to Riyadh for medical treatment, and Vice-President Abedrabbo Mansour Hadi assumed nominal control.
Yemen's constitution allows for the emergency transfer of power to the vice-president for up to 60 days, but the 60-day deadline came and went at the start of August, and Mr Saleh continued to insist that he was still in charge.
As the summer wore on, negotiations over the transition of power were sliding into stalemate. Instead of outright confrontation, competing factions took part in proxy clashes in Arhab - just north of Sanaa airport - the highland city of Taiz and Abbyan, a southern coastal province.
By late August, at the end of Ramadan, Mr Saleh was still recuperating in Riyadh.
His repeated promises to return to Sanaa, which had kept Yemenis on tenterhooks for months, had come to nothing.
But Mr Saleh's son, Ahmed Ali, and Ahmed Ali's three cousins, Tarik, Amar and Yahya - who control the Republican Guard and other elite security and intelligence units - remained embedded in the presidential palace in Sanaa.
Tensions began to rise again in early September.
On 12 September, Mr Saleh issued a decree granting his deputy, Mr Hadi, authority to negotiate a transition deal.
The international community welcomed the move, and the US government confidently voiced expectations that arrangements for a "peaceful and orderly transition" would be agreed within a week.
However, Mr Saleh's opponents were sceptical, sensing more delaying tactics.
The ensuing days saw renewed clashes in Sanaa between security forces under the control of Mr Saleh's family and the Ahmar brothers.
On Sunday, protesters - determined to break the stalemate over the transition talks and maintain momentum for change - marched outside the boundaries of their camp, along Zubayri Street.
Gunmen under the control of Mr Saleh's family opened fire on the protesters on Zubayri Street, killing 26 people and injuring many more.
Sunday's assault provoked immediate retaliation from Gen Ali Mohsin, who had pledged to protect the protest camp, known as Change Square.
Clashes between units under the control of Mr Saleh's family and Gen Ali Mohsin's division continued on Monday, centred around Zubayri Street and Change Square. At least another 20 people have been killed by security forces in the continuing crackdown.
Reports on Twitter suggested Ali Mohsin's division was also trying to push south towards the presidential palace.
Fighting has closed off one of Sanaa's main arterial roads, creating traffic jams and extended queues at petrol stations, as drivers anticipate future shortages and price rises.
Monday's violence came as the UN Human Rights Council in Geneva condemned the excessive use of lethal force by security forces under the control of Mr Saleh's family.
A report by the UN Office of the High Commissioner for Human Rights, issued last week, noted that Yemen's authorities "appeared to have lost effective control of parts of the country and within the major cities" and warned that Yemen was confronted by the prospect of civil war.