Ahmadinejad did not directly address the protests, but said his government would "resist any violation of law and interference".
"We will not remain silent, we will not tolerate disrespect, interference and insults," he said.
He criticised Western nations, several of which - including the United States, Britain, Italy, Germany and France - had declined to offer formal congratulations on his re-election.
"We heard that some of the Western leaders had decided to recognise but not congratulate the new government ... well, no one in Iran is waiting for your messages," he said.
Mehrdad Khonsair, a former Iranian diplomat told Al Jazeera: "He feels that this snub [from Western leaders] is such that he's had to respond ... it shows obvious disappointment.
"He essentially pointed out that negotiation with him in the future is going to be difficult."
Khonsair said that Ahmadinejad also sought to highlight the backing that he enjoys amongst the dominant political powers in Iran.
"He was saying he has the support of the important elements in Iran needed to impliment his policies for the next four years," he said.
Hundreds of police and members of the pro-government Basij militia had been deployed outside parliament in an apparent attempt to stop any demonstrators from gathering, witnesses said.
Al Jazeera correspondent Alireza Ronaghi said: "There was an amazing number of police officers on the streets.
"There was not even enough space on some of the sidewalks for people to walk because there were so many ... anyone who was even thinking of protesting would have been overwhelmed."
One witness told the AFP news agency that security forces used pepper spray against protesters in Baharestan square, near to the parliament building, as Ahmadinejad was sworn in.
"The protesters were chanting anti-Ahmadinejad slogans. The police and the Basijis dispersed them. All the nearby shops are closed," the witness said.
Another told the Reuters news agency that hundreds of protesters were walking around the parliament building.
"But there is no clash," the witness said. "Mobile phones have been cut off."
The White House, which Iran has repeatedly accused of interferring following the presidential polls, finally announced on Tuesday that it considered Ahmadinejad to be Iran's elected leader.
|About 2,000 people have been detained during protests against the election [AFP]
The dispute over the election has exposed divisions in Iran's ruling elite, with Ayatollah Ali Khamenei, Iran supreme leader, endorsing the poll and backing Ahmadinejad, while other leading political figures supported opposition calls for a re-run.
Senior officials and religious leaders attended the ceremony, which was boycotted by opposition leaders and some politicians.
Ayatollah Mahmoud Hashemi Shahrudi, the head of Iran's judiciary, had urged the country's rival groups to unite.
"I hope they do what is best for the revolution, the regime and Islam and disappoint the enemies," the ISNA news agency quoted him as saying.
Mir Hossein Mousavi, who came in second in the presidential race, and Mehdi Karroubi, another defeated reformist candidate, have both vowed to continue their protests against Ahmadinejad's election.
Ahmadinejad has two weeks to present a cabinet of ministers to the conservative-dominated assembly for approval following his inauguration.