Military intervention in Bahrain is “unacceptable”, said Iran’s foreign ministry spokesperson Ramin Mehmanparast.
"The presence of foreign forces cannot be acceptable and will make the situation more complicated and difficult," Mehmanparast told his weekly press conference broadcast on state television.
On Monday, television footage showed convoys of unmarked, desert-brown armoured vehicles crossing from Saudi's Eastern Province into Bahrain. The Saudi government says it was sending in troops in a response to a call from Bahrain for help.
"The council of ministers has confirmed that it has answered a request by Bahrain for support," the Saudi government said in a statement.
Bahrain has been rocked by clashes and protests inspired by upheavals in North Africa last month. On Sunday, dozens of people were injured in skirmishes with police in the worst violence since last month.
The opposition alliance said any foreign force would be treated as an invading army.
Shia Muslims, who have joined the opposition alliance, complain of discrimination in the Sunni-ruled kingdom.