Tehran, Iran (CNN) -- Iranian students stormed the British Embassy in Tehran Tuesday, breaking down the door, throwing around papers and replacing the British flag with an Iranian one.
Iran's state-run Press TV reported that police had the incident under control, but Britain's Foreign Office called the situation "fluid" and said "details are still emerging."
"There has been an incursion by a significant number of demonstrators into our embassy premises, including vandalism to our property," the Foreign Office said. "We are outraged by this. It is utterly unacceptable and we condemn it."
The Foreign Office added that "under international law, including the Vienna Convention," the Iranian government has "a clear duty to protect diplomats and embassies in their country."
The office said it asked the Iranian charge d'affaires in London "to urge the Iranian authorities to act with utmost urgency to ensure the situation is brought under control."
It is now advising British nationals in Iran "to stay indoors, keep a low profile and await further advice."
The Russian Foreign Ministry also condemned the protesters' actions.
"We are expressing our support for the British diplomats. We hope that the Iranian authorities will take necessary measures to immediately restore order, investigate the incident and prevent a repeat of such incidents," the ministry said in a written statement.
The incursion happened during a protest demanding that the British ambassador be sent home immediately.
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A crowd of about 1,000 people gathered near the embassy for the anti-British demonstration, and had been peaceful before some participants stormed the building. A CNN producer saw students throwing stones at the embassy's windows.
Miles away from the embassy, security forces closed the gate of a British diplomatic compound called Gholhak Garden to prevent students from entering, said Iran's official Islamic Republic News Agency, or IRNA. British diplomats and their families and Iranian families who work for the British Embassy live there.
The Iranian Parliament voted Sunday to expel the ambassador and reduce diplomatic relations with the United Kingdom in retaliation for newly imposed Western sanctions, according to IRNA. The vote must still be approved by the Guardian Council, the body that determines if laws passed by Parliament are compatible with Sharia law, or Islamic law.
The Foreign Office called the Iranian Parliament's vote Sunday "regrettable."
"This unwarranted move will do nothing to help the regime address their growing isolation or international concerns about their nuclear program and human rights record," the office said in a written statement. "If the Iranian government acts on this, we will respond robustly in consultation with our international partners."
Britain cut all financial ties with Iran last week over concerns about Iran's nuclear program, the first time it has cut an entire country's banking sector off from British finance, the British Treasury announced.
Last week, all British credit and financial institutions were ordered to end their business relationships and transactions with all Iranian banks, their branches and subsidiaries by Monday.
The move came after an International Atomic Energy Agency report highlighted new concerns about "the possible military dimensions of Iran's nuclear program," the Treasury statement said Monday.
"The IAEA's report last week provided further credible and detailed evidence about the possible military dimensions of the Iranian nuclear program," British Foreign Secretary William Hague said in a statement Monday. "Today we have responded resolutely by introducing a set of new sanctions that prohibit all business with Iranian banks."
Iran insists its nuclear program is peaceful and has called the U.N. watchdog's report "unbalanced" and "politically motivated."
The British sanctions underline "the severity of the government's concerns about Iran's activities," Britain's chancellor of the exchequer said.
The chancellor's statement said other "partner countries" will make similar announcements about banking sanctions against Iran.
Iran's proposal mandates the Foreign Ministry to adopt similar positions against any other countries that impose the same policies as Britain, IRNA reported.