UK Foreign Secretary Philip Hammond said he was "delighted" to be in the Iranian capital for the reopening of the embassy, in what is the first British ministerial visit to Iran in more than a decade.
Iran also reopened its embassy in London on Sunday in a coordinated move that reflects improved ties between the two nations.
Speaking at a joint news conference, Hammond and Iranian Foreign Minister Mohammad Javad Zarif both appeared hopeful that a turning point had been reached in their countries' relations.
Zarif said that after a period of "problematic relations," diplomats in the two embassies would now start working "to open a new chapter in relations between us and Britain."
It's a relationship that has gone through many ups and downs in recent years, he said, but will now enter a new phase "based on mutual respect and constructive engagement and dialogue," even in areas of disagreement.
Hammond: Work to tackle common challenges
Hammond said discussions on the sidelines of the nuclear talks and last year's U.N. General Assembly had shown what could be gained from direct dialogue, but acknowledged that there would continue to be areas where the two nations disagree.
"Iran is, and will remain, a very important country in a strategically important but volatile region. Maintaining dialogue, even under the most difficult conditions, is crucially important," he said.
Hammond said Britain would work to ensure that the nuclear agreement
is a success, including by making sure that it is fully implemented by all sides.
Britain and Iran should also work to tackle common challenges together, he said, including terrorism, regional stability, the spread of ISIS in Syria and Iraq, the illegal drug trade and migration.
Zarif sounded a similar note, saying that both sides were aware of the need to fight extremism, terrorism and sectarianism in the region. The two nations will also look to develop joint economic interests, he said.
Zarif: Relationship with the U.S. is different
Zarif appeared to dash hopes of a similar thaw in U.S.-Iran relations, however.
"Our relations with the United States are different," he said in response to a reporter's question, adding that the Iranian people have "considerable concerns" over U.S. policies.
These put Iran "in a different situation with the United States than with other countries, such as Britain, where we had problems but have been able to return relations to where they were previously," he said.
Asked about the continued opposition to the nuclear deal voiced by many in Washington, Hammond said it was always understood the agreement would be received differently there than in Britain, France or Germany, where it has been widely welcomed.
But he said he was confident that the United States would "come to the right decision" and endorse the deal.
In an earlier Foreign Office news release, Hammond said that while the attack in 2011 was a "low point," the relationship between Britain and Iran had steadily improved since Hassan Rouhani became president in 2013.
Rouhani's election is widely seen as having launched a more positive tone in Iran's interactions with the West.
Protest sparked by sanctions
Hammond was joined for the opening ceremony by the new UK charge d'affaires, Ajay Sharma, and representatives of the Iranian Ministry of Foreign Affairs and wider diplomatic community, the UK Foreign Office said.
The embassy in Tehran will operate with a small staff and offer a limited range of consular services at first, Hammond said. But an agreement on upgrading to full ambassador status is expected in the coming months, along with an increase in services.
A small trade delegation accompanied Hammond to Tehran.
The assault by student protesters on the UK Embassy and a separate diplomatic compound in Tehran in November 2011 prompted outrage and led Britain to close the embassy's doors and withdraw all its staff from Iran.
Britain also closed the Iranian Embassy in London and ordered all Iranian diplomats to leave.
The protest in Tehran was sparked by anger at UK sanctions imposed against Iranian institutions over the nation's nuclear program. The embassy buildings should have been guarded by Iranian security officers.