A senior aide to the former Lebanese prime minister Saad Hariri has been assassinated by a large car bomb in central Beirut. Fifteen other people were injured in the blast, which destroyed part of a neighbourhood near Hariri's compound.

Hariri-linked media reported that Mohamad Chatah, a senior adviser to the now exiled leader, died in the blast. He was believed to have been en route to a meeting at the nearby headquarters, where he kept an office. Chatah was an outspoken critic of the Syrian regime and of Hezbollah, which has held sway over the Lebanese government since Hariri was ousted as leader three years ago.

Chatah, 62, is the second senior opposition figure to have been killed in the past 14 months. The political killing of figures linked to the Hariris dates back nine years.

In October last year, Wissam al-Hassan, the head of the Internal Security Forces intelligence branch, was also killed by a car bomb. He was buried several hundred metres from the scene of Friday's blast in a shrine alongside former prime minister Rafiq Hariri, the patriarch of the western-leaning 14 March alliance whose assassination in February 2005 sparked a new era of instability in post-civil war Lebanon.

Friday's bombing comes several weeks before the start of the long-delayed trial of the alleged assassins of Hariri, five members of Hezbollah, who will be tried in The Hague in absentia. Hezbollah has vehemently denied being responsible for Hariri's death, which it labels as a US and Israeli plot.

A giant mushroom-like cloud towered over the downtown district where the explosion hit, not far from the Hariri compound in the Wadi Abu Jamil area. The Lebanese Red Cross said at least 15 casualties had been taken to nearby hospitals. It was not immediately clear how many had been killed and whether the toll would rise.

14 March leaders have been strong supporters of the uprising against Syrian leader Bashar al-Assad, whom they accuse of seeking hegemony over Lebanon directly and through Hezbollah and Iran. The bloc is strongly backed by Saudi Arabia and supported by Qatar. Each side is arming proxies in the vicious war in Syria, while seeking to assert its influence on the Lebanese political scene, which has been unable to form a government for more than a year.

Chatah had adopted an unusually strident tone against Syria and Hezbollah for the past year, taking to Twitter regularly to warn of the perils of political chaos and the influence of foreign players. "As Hezbollah chips deeper into the state's sovereign prerogatives, it undercuts the foundations of a single/united Lebanon," he said recently. In another tweet he said: "Arafat, then Assad then Nasrallah. If Lebanon is not saved from its current path, history will tell how the third blow led to its downfall."

In his final tweet, hours before he died, he said: "Hezbollah is pressing hard to be granted similar powers in security and foreign policy matters that Syria exercised in Lebanon for 15 years."

Britain's ambassador to Lebanon, Tom Fletcher, paid tribute to Chatah on Twitter. He said: "Mohamad Chatah was a wise, tolerant, smart patriot. His courage not that he knew risks but that he believed Lebanon worth taking them for.

"We condemn his cowardly assassination. Determined to honour his memory by support to justice, rule of law and stability for Lebanon."