France has raised its security alert level to "orange" in response to the train bombings in Madrid on Thursday.
French flags have been lowered to half-mast in tribute
The measure, which will see soldiers deployed to reinforce police and higher security at transport hubs, was decided at a meeting of top ministers.
The heightened alert "aims to guard against the risk of a terrorist or criminal action deemed plausible", said the prime minister's office.
Both Spain's neighbours Portugal and France have stepped up border controls.
The French Vigipirate four-level alert system ranges from the lowest level, yellow, to orange, red and scarlet.
The alert, which was reduced to yellow on 26 January, has now returned to orange after Thursday's multiple railway bombings in the Spanish capital, which killed almost 200 people.
The decision was taken as President Jacques Chirac met ministers for defence, justice, the interior, foreign affairs and transport, to review security on Friday morning.
In response, police will increase spot checks at railway stations, airports and maritime ports and troops will reinforce the police presence.
A recent bomb scare in France meant 32,000km of track had to be inspected
Police began conducting checks on all vehicles along the French border with Spain on Thursday night, according to Reuters news agency, paying particular attention to heavy goods vehicles.
The Spanish government has said the prime suspect in the attacks is the armed Basque separatist group Eta.
In the past, Basque separatists have been suspected of trying to escape the eye of Spanish authorities by crossing over the border into the French Basque region.
Tighter precautions are also being taken on the border with Portugal, a quarter of whose imports come from Spain.
France paid tribute to its traumatised neighbour on Friday.
President Jacques Chirac has ordered flags on public buildings to be flown at half-mast for three days.
And Prime Minister Jean-Pierre Raffarin will travel to Madrid on Friday to attend a protest demonstration led by the Spanish prime minister.
A rally in Paris on Friday evening has been called by French trade unions, to express solidarity. Many of the victims of Thursday's bombs were working people and school children.
Friday has also been declared a day of mourning in Portugal.