PARIS — Prime Minister Manuel Valls declared Saturday that France was at war with radical Islam after the harrowing sieges that led to the deaths of three gunmen and four hostages the day before. New details emerged about the bloody final confrontations, and security forces remained on high alert.

“We are 99 percent sure that she traveled to Syria from Urfa,” said a Turkish intelligence official, referring to a city in southern Turkey. “There is no evidence that suggests she was involved in the terrorist attacks in France this week.”

France remained on edge a day after security forces killed Mr. Coulibaly, who the police said was responsible for the deaths of four hostages at a kosher supermarket near the Porte de Vincennes in eastern Paris on Friday, and Saïd and Chérif Kouachi, the brothers who fatally shot 12 people on Wednesday in and around the offices of Charlie Hebdo, a satirical newspaper.

The French government said it would put 500 additional troops on the streets over the weekend amid preparations for a giant unity rally in Paris on Sunday. A number of European officials said they would attend, including Chancellor Angela Merkel of Germany, Prime Minister David Cameron of Britain and Prime Minister Ahmet Davutoglu of Turkey, the most prominent Muslim leader scheduled to be there.

Other foreign officials who have said they would be at the rally include Palestinian Authority President Mahmoud Abbas and Prime Minister Benjamin Netanyahu of Israel.

On Saturday, hundreds of thousands of people marched in Paris, Toulouse, Nice and other cities in a show of solidarity, and rallies were held in places as far away as Madagascar, Tel Aviv and Bangui, Central African Republic.

Top ministers in the French government held an emergency session to discuss measures to prevent a repeat of the attacks, which shocked the country and raised questions about why law enforcement agencies had failed to thwart terrorism suspects well known to the police and intelligence services.

Some of the surviving hostages shared chilling accounts of their ordeals at the hands of heavily armed captors, who they said had seemed prepared to die as police forces amassed outside the kosher supermarket and a printing plant northeast of Paris that the Kouachi brothers had seized early Friday.

The crisis and its aftermath presented a major challenge to President François Hollande and his government, which are facing deep religious and cultural rifts in a nation with a rapidly growing Muslim population while simultaneously coping with the security threats stemming from Islamic extremists. Large numbers of French citizens have been traveling to Syria and Iraq to fight with the Islamic State, also known as ISIS and ISIL.

Mr. Hollande, appealing for unity, has warned against seeing Muslims as the enemy, and Mr. Valls called again on Saturday for citizens to join the rally planned for Sunday.

“There needs to be a firm message about the values of the republic and of secularism,” Mr. Valls said in Évry. “Tomorrow, France and the French can be proud. Everyone must come tomorrow.”

But the event was already proving polarizing in some quarters. Marine Le Pen, the leader of the far-right National Front, which was not invited, urged her followers to stay away, saying the rally had been “taken over by parties which represent what the French hate: partisan spirit, electioneering and indecent polemic.”

Mr. Valls’s statement that France was at war with jihadists and radicals reflected the depth of the concerns over security in France and across much of Western Europe.

American officials said Attorney General Eric H. Holder Jr. would attend a meeting in Paris on Sunday convened by the French interior minister. Terrorist threats, foreign fighters and violent extremism were expected to be at the top of the agenda.

Simone Rodan Benzaquen, the director of the American Jewish Committee in Paris, said there was deep anxiety among French Jews, who had already been grappling with a spate of anti-Semitism in recent months, prompting some to declare their intention to leave France.

“We wonder whether it is the beginning of a series of further attacks or if it is the end, or a wake-up call that will help the country understand what Islamic radicalism is,” Ms. Rodan Benzaquen said.

The Defense Ministry said it had added 250 soldiers to the 850-person force already deployed in the greater Paris area, and would add 250 more on Sunday. Security forces could be seen outside vulnerable targets in Paris.

Yet a Turkish official said that French authorities were slow to provide information, allowing Ms. Boumeddiene to slip into Syria before she could be questioned about Mr. Coulibaly and the attacks.

Much remained unknown or unclear about the attacks, the suspects and the efforts to hunt them down. After initially suggesting that the Kouachi brothers had another accomplice, the police have said little since an 18-year-old with the name of the person they were seeking turned himself in late Wednesday.

The precise nature of the relationships among the Kouachi brothers, Mr. Coulibaly, the Islamic State and another terrorist group, Al Qaeda in the Arabian Peninsula, remained murky. And with security services still on high alert, there was little public indication of whether the authorities had specific information about possible further attacks or simply remained worried about the potential for other terrorists to take cues from the French gunmen and from the jihadists and militant groups that had quickly applauded them as heroes.

Le Monde, citing counterterrorism officials, said that the links between Mr. Coulibaly and Chérif Kouachi appeared to stretch back to 2010, and that telephone wiretaps showed that they had frequently visited Djamel Beghal, a French-Algerian champion of jihad who had once been imprisoned for planning an attack on the American Embassy in Paris in 2001. The police said Mr. Coulibaly and Mr. Kouachi had been followers of Mr. Beghal.

Mr. Coulibaly was an associate of Chérif, the younger Kouachi brother, a sometime pizza delivery man and fishmonger.

The brothers were being tracked by intelligence officials in the United States, France and Yemen, but large gaps remain in the chronology of their actions and in accounts of their affiliations. A senior United States official said on Saturday that Saïd Kouachi had traveled to Yemen with another person, probably an accomplice, in the summer of 2011.

The authorities are trying to determine whether that person was Chérif, who claimed to have gone to Yemen that year.