Hadi, who took office in 2012 under a UN- and Gulf-backed peace plan, is a career soldier with no popular or tribal base but who emerged as a consensus figure.
The Huthi Shiites, accusing Hadi of reneging on a deal, seized the palace in Sanaa and attacked his separate residence in what a minister said was a bid to overthrow the president and his US-backed government.
The fate of Hadi, reportedly at his residence when it came under fire, was not immediately known but his authority had already been undermined by the Huthis' unopposed takeover of Sanaa in September and by Yemen's chronic instability.
Since their advance from their power base north of Sanaa, the Huthis have pressed on into areas south of Sanaa, where they have met deadly resistance from Al-Qaeda loyalists.
Hadi's government has been a key ally of the United States, allowing Washington to carry out repeated drone attacks on Al-Qaeda militants in its territory.
Taking over from veteran strongman Ali Abdullah Saleh, who stepped down after an almost year-long and often bloody uprising, Hadi pledged to "preserve the country's unity, independence and territorial integrity".
A major general from restive southern Yemen, Hadi served as Saleh's vice president for 18 years and remained secretary general of the ruling General People's Congress party until its political bureau, led by the former strongman, announced Hadi's dismissal in November.
Saleh, who stepped down in 2012, accused his longtime aide of soliciting UN sanctions against him.
Hadi had never played a top role in politics before taking over Saleh's powers in June 2011 when the latter was wounded in an attack on his presidential compound.
And he was a crucial player in convincing Saleh to sign the UN-backed transition plan in late 2011.
Signup to our newsletter and follow us on Facebook and Twitter!
But during the transition period, Hadi struggled to untangle the deeply tribal and impoverished country from the grasp of Saleh, who had placed relatives and loyalists in top posts.
In his efforts to restructure the armed forces, as stipulated by the deal which brought him to power, Hadi frequently ran into opposition from Saleh's loyalists.
Born on May 1, 1945, Hadi graduated from a military academy in formerly independent South Yemen and also received military training in Britain and Egypt.
A unified Yemen was proclaimed on May 22, 1990, four years after Hadi had joined the northern camp.
The southerners tried to break away in May 1994, sparking a bloody civil war during which Hadi was appointed defence minister.
As president, Hadi tried to gain the support of southern separatists by reinstating hundreds of senior army and intelligence officers sacked after the civil war and ordering compensation for those whose properties were confiscated.
Like the Huthis, the southerners rejected plans to divide Yemen into a six-region federation but they appeared at times conciliatory towards Hadi.
But his relationship deteriorated with the Huthis who challenged his authority, seizing the capital and other towns, and then abducted his chief-of-staff, Ahmed Awad bin Mubarak, on Saturday.
Hadi has two daughters and three sons, and has written several books, including one on the military defence of mountain areas.