Iraq has cancelled police leave and put all officers on high alert ahead of a withdrawal of US combat forces.
US troops are due to pull out of Iraqi towns and cities at the end of the month.
The US pullback from urban centres has been seen as a milestone on the country's road to sovereignty.
But a string of bombings in Baghdad and northern Iraq this week, including two of the deadliest attacks for more than a year, have shaken the confidence of Iraqis in their own forces.
Security was tightened across the capital today, with troops and police closing roads and carefully searching cars.
'The alert has gone to all of our forces. There will be no days off. They are at their full strength across the whole country, at 100%,' said Major General Abdul-Karim Khalaf, spokesman for the interior ministry.
'All of our units have seen an increase in their numbers, not only at the checkpoints,' he said.
Prime Minister Nuri al-Maliki said yesterday that the US withdrawal sent a message to the world that Iraq could handle its own security.
Two big bombings in Baghdad and near the northern city of Kirkuk in recent days killed more than 150 people between them.
On Friday, a bomb killed at least 13 people at a Baghdad market.
US and Iraqi officials have warned they expect the number of attacks to increase as the US troops pull back, and also in the run-up to a parliamentary election next January.
