The Portuguese state weather agency on Friday put the capital Lisbon on high weather alert for the weekend, with temperatures expected to reach 41 degrees Celsius (105.8 Fahrenheit) on the last day of Pope Francis' visit.
Francis arrived in Lisbon on Wednesday for the week-long World Youth Day, a gathering of hundreds of thousands of Catholics that takes place every three years in a different city. The event will close with a papal Mass on Sunday.
The Mass will be celebrated on the riverside Parque Tejo, a large, shadowless outdoor area, and authorities said over 1 million people would attend.
World Youth Day participants were given what organisers have called the "pilgrim kit", which included a hat and a reusable water bottle.
The weather agency IPMA issued a "red" alert for Lisbon between 10 a.m. and 6 p.m. time (0900-1700 GMT) due to "persistence of extremely" high temperatures. The Sunday Mass will last two hours from 9 to 11 a.m.
Before the closing event, 86-year-old Francis will travel to Portugal's famous Catholic sanctuary of Fatima on Saturday morning to pray the rosary with sick youngsters, people with disabilities and inmates.
Francis will hold a vigil with young people at Parque Tejo on Saturday evening.
IPMA said temperatures on Saturday could rise to 36 degrees Celsius and 38 degrees Celsius in Fatima and Lisbon, respectively. August is usually the hottest month of the year in Portugal. The average highest temperature in August in Lisbon is around 28 degrees Celsius.
Bracing for a sweltering weekend, worried faithful taking part in a Via Crucis (Way of the Cross), which retraces the last hours in Jesus' life, called for action to tackle climate change.
Extreme weather in July has caused havoc across the planet, with record temperatures in China, the United States and parts of southern Europe sparking fires, water shortages and a rise in heat-related hospital admissions.
Portugal is on high alert for blazes, and more than 500 firefighters are scrambling to put out flames that started on Friday afternoon in a forest area in the central Castelo Branco district.
