Analysts believe reform will stall if the issue drags into 2008, as focus turns to the race for the White House.
Bush's proposals include the creation of a temporary guest worker programme and clearing the way for some undocumented immigrants to become US citizens, coupled with a series of tougher security measures.
Divisions among activist groups about the best way to campaign for reform as well as fear among illegal workers following a series of immigration crackdowns led to a lower turnout, activists say.
But in Chicago, many protesters said they were motivated by a high-profile raid by heavily armed FBI agents in the heart of one of the largest Mexican neighbourhoods in the Midwest.
|This year's marches in Los Angeles were much|
smaller than those of 2006 [AFP]
In Chicago, police put the size of the demonstration at around 150,000, while organisers said one million protesters marched through the heart of the city waving US flags.
In Los Angeles, where around 500,000 protesters brought the city's business district to a halt last year, police said between 8,000 to 10,000 people had gathered for a march to city hall.
Route to citizenship
Waving US and Mexican flags, marchers called for reform to offer a route to citizenship for illegal immigrants.
Many carried placards reading "Legalization Now!" and chanted "stop the deportations."
|The foreign-born population of the US is now|
more than 33 million strong [AFP]
In Detroit, about 10,000 people, also waving Mexican and US flags, marched carrying banners reading "Legislation Now, No Guest Worker Bill" and "Amnesty for all".
"We're here for my late father who was an illegal," said Rosalie Delagarza Johnstone, a Detroit-born Mexican-American who attended the rally with her 18-year-old son, David, who skipped school to participate.
"He was deported, then fought his way back here to make a better life for me and my brothers and sisters.
"He joined the army and could have died, would have died, to help us."
Sea of colour
On the east coast, groups rallying under the New York Immigration Coalition and National May 1st Movement for Worker and Immigrant Rights marched to demand more rights for all undocumented immigrants.
Downtown Manhattan's Union Square became a sea of colour, with the Stars and Stripes mingling with flags from Honduras, Mexico, El Salvador and Ecuador.
The foreign-born population of the US is now more than 33 million, nearly 12 per cent of the total.