Saudi Arabia has bombed key military installations in Yemen after announcing a broad regional coalition to oust the Shiite rebels who forced the country's embattled president to flee.
Some of the strikes hit positions in the country's capital, Sanaa, and flattened a number of homes near the international airport.
The airstrikes, which had the support of nine other countries, drew a strong reaction from Iran.
It said the operation was an "invasion" and a "dangerous step" that will worsen the crisis.
Iran "condemns the airstrikes against Yemen this morning that left some innocent Yemenis wounded and dead and considers this action a dangerous step", Foreign Ministry spokeswoman Marzieh Afkham said.
She said military action would complicate and worsen the crisis in Yemen.
"This invasion will bear no result but expansion of terrorism and extremism throughout the whole region," she said.
The Saudi airstrikes came hours after President Abed Rabbo Mansour Hadi fled Yemen by sea after rebels pushed their way toward the southern port city of Aden where he had taken refuge.
The back-and-forth between the regional heavyweights was threatening to turn impoverished Yemen into a proxy battle between the Middle East's Sunni powers and Shiite-led Iran.
Saudi-owned Al-Arabiya News reported that the kingdom had deployed 100 fighter jets, 150,000 soldiers and other navy units in what it called Operation Decisive Storm.
The Shiite rebels, known as Houthis, were calling on their supporters to protest in the streets of Sanaa, Yemen's Houthi-controlled state news agency SABA reported.
TV stations affiliated with the rebels and their ally, former president Ali Abdullah Saleh, showed the aftermath of the strikes this morning in what appeared to be a residential area.
Al-Masirah TV, affiliated with the Houthis, quoted the ministry of health as saying that 18 civilians were killed and 24 were injured.
Yemen Today, a TV station affiliated with Saleh, showed hundreds of residents congregating around a number of flattened houses, some chanting "death to Al-Saud", in reference to the kingdom's royal family.
An Associated Press reporter in the Sanaa neighbourhood near the international airport saw people searching for loved ones in the debris of flattened homes.
Residents said at least three bodies were pulled from the rubble. There were traces of blood between the bricks.
Ahmed al-Sumaini said an entire alley close to the airport was wiped out in the strikes overnight. He said people ran out from their homes in the middle of the night.
"This was a surprise. I was asleep and I was jolted out of my bed," he said, waving a piece of shrapnel.
Targets also included the camp of Yemeni special forces, which is controlled by generals loyal to Saleh.
Yemeni security officials said the targets also included a missile base in Sanaa that was controlled by the Houthis earlier this year. One of the security officials said the strikes also targeted the fuel depot at the base.
The Houthis said Saudi jets hit the military base, known as al-Duleimi, and that they responded with anti-aircraft missiles.
The strikes also hit the al-Annad air base in the southern Lahj province. About 100 US military advisers withdrew over the weekend from that base, where they had been leading a drone campaign against al Qaida in the Arabian Peninsula.
Riad Yassin, Yemen's foreign minister, told Saudi's Al-Hadath TV that the airstrikes were welcomed.
"I hope the Houthis listen to the sound of reason. With what is happening, they forced us into this," he said.
Saudi ambassador to the United States Adel al-Jubeir said his government had consulted closely with the US and other allies but the US military was not involved in the operations.
The White House said the US was coordinating military and intelligence support with the Saudis but not taking part directly in the strikes.
The United Arab Emirates, Kuwait, Qatar and Bahrain joined Saudi Arabia in a statement saying they would answer a request from Hadi "to protect Yemen and his dear people from the aggression of the Houthi militias which were and are still a tool in the hands of foreign powers that don't stop meddling with the security and stability of brotherly Yemen".
Oman, the sixth member of the Gulf Cooperation Council, did not sign the statement.
Egypt also announced political and military support, saying it is ready to send ground troops if necessary. Jordan confirmed it was participating in the operation. Pakistan, Morocco and Sudan were also taking part, the Saudi Press Agency reported.
Saudi Arabia and its Gulf allies believe the Houthis are tools for Iran to seize control of Yemen and say they intend to stop the takeover. The Houthis deny they are backed by Iran.