1,000 extra police guard Paris targets

After the blast: Travellers are warned to be vigilant as intelligence source links gas-canister attack to Islamic extremists

Paris was on high alert yesterday as anti-terrorist measures, familiar from last summer, came into force after the rush-hour bomb explosion on Tuesday at Port Royal station. An additional 1,000 police and gendarmes were drafted in to patrol potential targets and 700 troops were placed at the city's disposal.

Two people were killed in the explosion and nearly 100 more were injured.

The most overt security was at stations and tourist sites such as the Eiffel Tower and the Louvre precinct, where gendarmes and CRS riot police patrolled in pairs. Security announcements were made through the day at underground stations, where passengers were handed small blue and white cards headed "Vigilant, together".

The impression was of a more discreet, but also more discriminating, security presence than last summer, and of a French public quietly returning to well-rehearsed routines.

In such circumstances it seemed almost superfluous for a government spokesman to call for "calm and sang-froid". But ministers revelled in the opportunity to call for national solidarity with the government to combat terrorism and they had the gratifying spectacle of trade unions cancelling planned protests because of the attack.

Outside Paris, security was increased in Bordeaux, where the Prime Minister, Alain Juppe, is mayor, and in Lyons, which was a target during last summer's bomb campaign. Suburban housing estates known to have large concentrations of first- and second-generation north African immigrants and active Islamic groups were placed under special surveillance. There was additional security at airports and borders.

Answering questions in the National Assembly, after a minute's silence for the victims of the bomb, Mr Juppe said no one had so far admitted responsibility and no one line of inquiry was being preferred.

The French news agency AFP, however, quoted "informed sources" - likely to be the intelligence service - as saying that a warning had been out since early November that a bombing campaign could be expected.

They cited an Islamic newsletter from September and a separate tip-off that a commando unit was about to enter France from Italy after training in the Middle East.

The type of bomb - a 13kg gas canister filled with explosives and 10cm nails which was said by experts to have caused as much damage as a grenade - and the timing and location of the attack had immediately recalled last year's campaign by Algerian Islamic terrorists.

Despite these parallels, the foreign ministry spokesman, Jacques Rummelhardt, said that there was so far "nothing to link the attack with the situation in Algeria... we are still at the stage of hypotheses". Some observers connected the bomb with last weekend's referendum in Algeria which had supported a new constitution outlawing religious parties.

Others noted, however, that France's first big terrorist trial of recent years is due to open in Paris on Monday. The case concerns the bombing of a hotel in Marrakesh in Morocco in 1994.

Two French citizens of north African origin are in prison in Morocco after being convicted of the attack, but the inquiry also uncovered a recruiting network based in France.

The case provided the first evidence of Islamic fundamentalist recruitment on French housing estates - a pattern repeatedly uncovered during the investigation into last year's bombs in France.