British ambassador returns to Iran after arrest last month
Macaire has now returned to Tehran amid speculation that the arrest might result in him leaving his post.
"I recently returned from London to Tehran. This was an important trip that was planned some time ago," Macaire said in Farsi in an Instagram video, according to Reuters.
While in the UK, the ambassador met with Britain's foreign minister and other top officials.
In response, the Islamic republic called "for an immediate halt to all interference and provocation by the British embassy" and warned that Iran's response "will not be limited to summoning the ambassador (to the ministry) if this attitude persists".
Iranian authorities confirmed Macaire's arrest as a foreigner at "an illegal gathering" but said he was released soon after being identified.
Macaire said he went to an event advertised as a vigil and left after five minutes when some people started shouting. He was arrested half-an-hour later.
Macaire was appointed ambassador to Tehran in 2018. As a diplomat, Macaire should be immune to arrest or prosecution but can be expelled by the Iranian authorities.
Mourning for the victims of crashed Ukraine International Airlines flight PS752 turned to angry demonstrations in January after Iran's Revolutionary Guards Corps admitted the plane had been shot down by a surface-to-air missile fired in error.
The plane crash occurred just hours after Tehran fired a volley of ballistic missiles at two bases in Iraq hosting US forces. That attack came in retaliation for the killing of top Guards general Qasem Soleimani in a US drone strike last week.
Read more: The legitimacy crisis and upcoming parliamentary elections in Iran
All 176 people on board the flight were killed, many of them Iranian citizens or Iranian-Canadian dual nationals.
Speaking to Reuters after the incident, Iranians condemned the government's earlier denials of culpability over the crash.
"Unintentionally? What does it mean? They concealed this huge tragic news for days just to mourn for Soleimani. Shame on you," said Reza Ghadyani from Tabriz.