BENGHAZI, Libya — Moammar Gadhafi took advantage of international indecision Saturday to attack the heart of the 5-week-old uprising, sending troops, tanks and warplanes to swarm the first city seized by the rebels. Crashing shells shook buildings, and the sounds of battle drew closer to Benghazi’s center.

“Where is France, where is NATO?” cried a 50-year-old woman in Benghazi, where a doctor said 27 people were killed Saturday. “It’s too late.”

Leaders from the Arab world, the United States and other Western powers are holding urgent talks in Paris over possible military action, and France’s ambassador to the United Nations, Gerard Araud, told BBC Newsnight that he expected military action to begin within hours of the meeting.

In an open letter, Gadafhi warned: “You will regret it if you dare to intervene in our country.”

On Saturday, a warplane was shot down over the outskirts of Benghazi, sending up a massive black cloud of smoke.

Before the plane went down, journalists heard what appeared to be airstrikes from it.

The fighting galvanized the people of Benghazi, with men collecting bottles to make gasoline bombs. Some residents dragged bed frames and metal scraps into the streets to make roadblocks.