LONDON - Rows over immigration and Europe overshadowed a strong performance by Gordon Brown on Thursday in his first cross-party grilling in parliament since becoming prime minister.

Brown displayed a sound grasp of domestic and foreign policy during two-and-a-half hours of questioning by senior lawmakers.

He said Britain would continue to press for sanctions against Iran over its nuclear programme and demanded Moscow reverse a decision to close British cultural centres.

But the timing of the appearance sparked controversy because it meant he arrived in Lisbon too late to sign a new European Union treaty with other leaders and had to sign it on his own at a restaurant later.

That led to accusations from opposition Conservatives -- denied by the government -- that he was ashamed of signing a document that Eurosceptics see as eroding British sovereignty.

Within minutes of Brown heading off to Lisbon, his government was again under attack from Conservatives who accuse the government of losing control of immigration.

Home Secretary (Interior Minister) Jacqui Smith told parliament more than 6,600 security industry employees in Britain did not have the right to work in the country -- more than initially estimated when the story broke last month.

The Conservatives accused her department of covering up a security blunder after it emerged some illegal immigrants could have been cleared to work as guards at government facilities.

Then an influential cross-party parliamentary committee rejected government plans to let police hold suspects without charge for 42 days from 28 now, saying the case for the measures was not convincing and the plans risked alienating Muslims.

When the ruling Labour Party tried in 2005 to extend the period, then Prime Minister Tony Blair suffered his first defeat in parliament. Brown now also faces a rough ride with the plans.

POLL SLUMP

Brown's performance earlier in parliament reassured members of his Labour Party who have been alarmed by its slump in the opinion polls following a scandal over secret donations and news the government had lost half the population's personal data.

It was the first time he had appeared before parliament's cross-party Liaison Committee, which subjects the prime minister to an intense grilling twice a year, since he took over from Tony Blair in June.

"He performed well and came across with authority," said Mike Gapes, a committee member from the ruling Labour Party.

Brown enjoyed a honeymoon with voters when he took over from Blair but after he abandoned plans to call an early election in October his government stumbled badly. Britain was also hit by its first bank run in more than a century.

Labour led the Conservatives in September by as much as 11 points in the polls, but is now behind by a similar margin. However Brown does not have to call an election before 2010.

"Basically the Labour Party is now back (in opinion poll standings) where it was before Blair announced his resignation," said John Curtice, politics professor at Strathclyde University.

"The trouble now is you've played your trump card (by changing leader)," he said.