SAN'A, Yemen - Troops commanded by relatives of Yemen's outgoing President Ali Abdullah Saleh attacked a crowd of more than 100,000 protesters peacefully marching into the capital Saturday, killing at least nine and driving Saleh to promise to leave the country.

Yielding to pressure to defuse the country's turmoil, Saleh said he would leave for the United States after forces overseen by his son and nephew opened fire on the protesters.

There was no immediate confirmation from American officials that Saleh would come to the United States, though Yemeni officials said he had received a visa.

In the first march of its kind in Yemen, an impoverished nation that is home to a dangerous al-Qaeda offshoot, the protesters traveled on foot for four days and 200 miles to pressure the government not to give Saleh immunity from prosecution.

After protesters arrived at the southern entrances to the capital, forces of the elite Republican Guard fired on them with automatic weapons, tear gas, and water cannons, sparking hours of clashes.

The violence illustrated the confusion in Yemen caused by Saleh's slow-motion exit from power after 33 years.

After entrenching for months against massive protests demanding his ouster, Saleh signed a deal in late November handing over his powers to the vice president, Abed Rabbo Mansour Hadi, and committing to leave office in return for immunity.

But Saleh retains his title and remains in the presidential palace, lauded as "his excellency the president" by state media controlled by his supporters.

Opponents say he has continued to wield influence through his relatives and loyalists who still hold powerful posts, undermining the vice president. That has raised fears that Saleh was looking for a way to keep ruling, even as a national unity government prepares for presidential elections in February.

The Republican Guard forces, led by Saleh's son Ahmed, and Central Security troops, led by his nephew, have defied orders from Hadi that they pull back from positions in the streets of San'a, even up to a Saturday deadline.

Saleh had been expected to leave Yemen soon after he signed the U.S.- and Saudi-backed deal, ostensibly to undergo treatment for wounds he suffered in a June assassination attempt. But officials say he has stalled on leaving.

Hours after the fighting erupted in San'a, Saleh told reporters at his palace that he would leave "in the coming days" for the United States.

"Not for treatment, but to get out of sight and the media to calm the atmosphere for the unity government to hold the presidential election," he said.

He said he would eventually return and pursue "political work as an opposition figure."

Government officials said ambassadors from the five permanent members of the U.N. Security Council had been in contact with Saleh in the last week to pressure him to leave. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

The protesters, who have been massing by the millions around Yemen since February, oppose an accord for Saleh's exit because it would grant him immunity and because it keeps many of his regime figures in place.

Saturday's "March of Life" demanded that he be put on trial for the killings of protesters during his crackdown on the uprising.