Looking fatigued in sweltering heat, Pope John Paul II (search) held up families as the bedrock of society Sunday as he celebrated Mass in this tranquil Adriatic seaport.

The 83-year-old pope, outside the Vatican on a Pentecost Sunday for the first time as he continued his 100th foreign pilgrimage, encouraged young Croats to marry, raise families and help their children overcome temptations to walk "the path that leads to authentic joy."

"You are an important part of society and the church," the pope told families among at least 100,000 pilgrims gathered at a river delta, some of whom spent the night lighting candles and praying.

The pope said he empathized with the struggle of families in this ex-Yugoslav republic to make ends meet, and he urged the country's politicians not to ignore their plight. Joblessness in Croatia (search) is about 20 percent, and the average monthly wage is just $450.

"It must not be forgotten that in helping the family, we also help to resolve other important problems, such as providing assistance to the sick and the elderly, stopping the spread of crime and finding a remedy to drug use," John Paul said.

As in much of Europe, many young couples in this overwhelmingly Roman Catholic nation tend to live together out of wedlock. The pope singled them out in his homily, saying they should not be afraid of "a bond which is publicly manifested and recognized."

Although his voice was clear and strong, the frail pontiff looked tired and drawn, shielded by a canopy from temperatures that were 90 degrees and climbing.

But John Paul -- nearing the end of a draining, five-day, five-city trip in often sweltering heat that was blamed for the deaths of two pilgrims Saturday in eastern Croatia -- has showed a stamina that has surprised even his Vatican entourage.

"He is amazing," said papal spokesman Joaquin Navarro-Valls. "What you can see everywhere is the inner strength of this man who keeps going without even asking: `Am I tired? Is this too much?' He just keeps the pace -- the pace he has imposed on himself."

The pope, who suffers from Parkinson's disease and crippling knee and hip ailments and has been wheeled around on a special hydraulic chair, seemed to draw strength from the euphoria that has greeted him on every stop. This is his third visit to Croatia in nine years.

"We are grateful, honored and proud to have him here for the third time," said Katica Marusic, 63, a retired saleswoman in Rijeka. "I hope he feels that."

The Vatican has been planning a papal pilgrimage in August to Mongolia (search), while trying to also persuade the Russian Orthodox Church to approve a stop in the Russian city of Kazan, where the pope would return an icon of the Virgin Mary. It would be the first visit ever by a pope in Russia, where the Vatican is seen as trying to expand its influence.

Navarro-Valls told reporters Sunday that "the trip is still evolving. Maybe the moment has not arrived. However, the trip has not been canceled," suggesting that the Vatican still hoped to get the Russian church on board for such a pilgrimage at a later date.

John Paul, who planned to visit the city's 700-year-old shrine of St. Mary of Trsat later Sunday, drew applause and cheers when he finally managed to correctly pronounce the tongue-twisting name "Trsat" in his homily after two stumbling attempts.

Sunday's Mass overlooked a river that once was the border between territories occupied by neighboring Italy and those belonging to Croatia. Today, Croats live alongside Rijeka's Italian and Serb minorities, and they did so even during the country's bloody 1991 war for independence.

Security was heavy for the pope's visit; Rijeka teemed with police, and helicopters made low sweeps over the city.

But Mayor Veljko Obersnel noted that Rijeka's residents "cherish their openness and tolerance," suggesting they were modeling the pope's exhortations to set aside wartime rivalries and coexist in peace.

John Paul was to meet later with Croatia's prime minister. Showing he keeps up with contemporary events, he also was to bless the cornerstone for the village of the 2009 Mediterranean Games, which Rijeka hopes to host.

The pope wraps up his trip on Monday with a visit to the southern coastal city of Zadar.