Huge explosions shook buildings in the heart of the capital Tuesday, apparently from U.S. forces bombing Iraqi positions on the city outskirts. With coalition troops closing in, the streets were nearly empty as people hunkered down in anticipation of the battle for Baghdad.

Thousands of Marines poured toward the city in convoys, taking dirt roads to avoid cities and towns and creating traffic jams in the push north.

A cold, howling wind blew gray smoke over Baghdad from fuel fires that Iraqi authorities started to conceal targets. Blasts could be heard in the distance.

The explosions started at midnight Monday, flashing a faint orange glow on the horizon to the south, where units of Saddam Hussein's Republican Guard are located. In Washington, Pentagon officials said Monday that U.S. helicopters had begun attacking Saddam's forces arrayed around the capital.

Baghdad's many dogs stopped barking during the explosions but the late-night Islamic call for prayer continued to sound from minarets.

At about the same time, in northern Iraq, heavy bombing was heard from near the key northern city Mosul, indicating that coalition forces were hitting positions closer to the border with the Kurdish area.

On Monday in Baghdad, security and police officers dug more trenches around military offices in the center of the capital, as smoke from fires set to conceal targets from bombing hung over the city.

Daytime traffic was heavy in some areas, youngsters played soccer on side streets and Iraqis walked the city despite the tension from days of bombing by coalition forces.

Some shops reopened along the commercial Al-Rasheed Street, but most were suitcase vendors. The upscale area of Irasat al-Hindiah, where Baghdad has its fashionable restaurants and boutiques, was nearly deserted.

Saddam tried to rally his people in a TV appearance Monday calculated to show that U.S. bombs and missiles had missed him. Iraq also claimed Monday to have shot down two American helicopters and taken pilots prisoner.

During a briefing at Camp As Sayliyah in Qatar, U.S. Commander Gen. Tommy Franks acknowledged an attack helicopter operating south of Baghdad was missing with its two-man crew.

The Iraqi government continued to urge citizens to resist invaders as the military prepared defenses against U.S.-led troops advancing on Baghdad.

Anti-aircraft guns that had been removed earlier were placed once again atop one of the main gates to the Old Palace, a presidential compound hit in earlier attacks. Information Minister Mohammed Saeed al-Sahhaf said the U.S. bombardment of Baghdad had injured 194 civilians.

Announcers on Iraq's two TV stations have started wearing olive-green military uniforms to introduce patriotic songs, archival footage of Saddam and old films with a patriotic message.

"You Iraqis are in line with what God has ordered you to do, to cut their throats," Saddam said in his television appearance.

Saddam looked strikingly more vigorous than he did in the speech that aired hours after the first airstrikes on Baghdad last week. Saddam referred specifically to U.S. tactics and the fighting around Umm Qasr in an obvious attempt to show that the address was current.

In Washington, a senior U.S. official speaking on condition of anonymity said U.S. intelligence had determined that Saddam's speech was recorded. However, it is unclear when it was taped.