President Mohamed Morsi condemned Tuesday's attacks on US diplomatic missions in Cairo and Benghazi during a press conference in Brussels on Thursday.

He went on to condemn the anti-Islam film that sparked the attacks, describing any insult against the Prophet Mohamed as a "red line."

"We Egyptians reject any kind of assault or insult against our prophet. I condemn and oppose all who... insult our prophet," Morsi said.

He offered his condolences on the death of Christopher Stevens, the US Ambassador to Libya, and three other embassy staff, who were killed late Tuesday during an attack on the US consulate in Benghazi.

Morsi went on to say that Islam strongly condemns the killing of innocent people. “Whoever kills a soul, it is as if he had slain mankind entirely," he said, quoting the Quran. 

He asserted that people had the right to protest peacefully and that Egypt was strong enough to protect foreign missions and envoys on its soil, as well as public and private property.

He warned against further attacks on foreign missions that would violate Egyptian and international law. 

Morsi said that he had spoken with US President Barack Obama and told him that it was necessary to put in place "legal measures which will discourage those seeking to damage relations... between the Egyptian and American people."

In Cairo, police used tear gas on Thursday as they clashed with a stone- and bottle-throwing crowd protesting near the US embassy in Cairo over a film mocking Islam, witnesses and the interior ministry said.

Protests against the film were also held on Wednesday outside US missions in Morocco, Sudan, Yemen and Tunisia. In Tunis, police fired tear gas to disperse a crowd of several hundred.

The low-budget movie, Innocence of Muslims in which actors have strong American accents, portrays Muslims as immoral and gratuitously violent.

It pokes fun at the Prophet Mohammed and touches on themes of paedophilia and homosexuality, while showing him sleeping with women, talking about killing children and referring to a donkey as "the first Muslim animal."

Mystery has deepened over the film, with conflicting accounts from backers and promoters but no one owning up to having actually directed it.

President Morsi is currently on official visits to Belgium and Italy in order to bolster Egypt-EU relations.